There're at least two ways...

The following let the barbarian actually rage using his own kind of rage more often than normal. "Whirling frenzy is otherwise identical to the standard barbarian rage" (UA 66), so these should work with that, too.

• The 2nd-level Drd spell blood frenzy [trans] (Spell Compendium 33) causes 1 touched creature to rage as if it used its special ability rage normally "but this rage does not count toward the creature’s number of rage attempts per day." Note: Rage attempts?

  This is likely the best choice, equipping an ally with a wand of blood frenzy (2nd-level spell at caster level 3) (90 gp/charge) and the barbarian himself with a potion of blood frenzy (2nd-level spell at caster level 3) (300 gp; 0 lbs.) in a potion bracer (Sharn: City of Towers 158, 159) (100 gp; 1.5 lbs.) for emergencies (that is, if the DM's not ready for the ready-drink helm to enter her campaign.)

• The shoulders slot item mantle of rage ("New Magic Items" 2) (6,000 gp; 1 lb.) grants the wearer, if a barbarian, the ability to rage 1 additional time per day, and, if not a barbarian, the ability 1/day to rage like a level 1 barbarian.

...And a clutch of edge cases

In addition to the 3rd-level Sor/Wiz spell rage [ench] (PH 268) et al. (which isn't really rage but a buff spell with attached penalties), there are a few lesser alternative sources of barbarian rage not involving taking levels or feats.

• The gutworm (Fiend Folio 220) is a symbiont that at will can cause the host to "enter a frenzied state identical to a barbarian’s rage" for 4 rounds, but this rage explictly provides bonuses like the typical level 1 barbarian's rage. Note: More rage, but not the right kind of rage.
• The face slot item mask of fury (Dragon #324 76) (12,000 gp; 2 lbs.) 2/day grants the wearer the ability to take either a free action (if he's been wounded within the last hour) or a standard action and deal himself 3 hp of damage (if he hasn't been wounded within the last hour) to enter a rage "as the barbarian ability." Note: Whether this can be a barbarian wearer's own rage or must be the rage of a level 1 barbarian is the DM's call.
• The supernatural ability past life (DMG2 159-60) has one option that 1/day can grant the possessor the ability to enter explicitly a greater rage as a level 11 barbarian. This special ability is nominally for NPCs and has a (ridiculous) Level Adjustment of +5. Note: Probably too late for this barbarian and too stupid for any PC (or, for that matter, any PC's follower or cohort).

Yes, that last scrapes the barrel's bottom.

A single-level dip in cleric can get you three things: the Inquisition domain with its +4 to dispel checks, Turn Undead that can power Divine Defiance, and the ability to activate a prayer bead of karma for a straight +4 bonus to CL.

Ur-priest does get you the last two things, but it’s difficult to enter and you need two levels instead of just one to get Rebuke Undead from it.

Anyway, Divine Defiance is a feat in Fiendish Codex II: Tyrants of the Nine Hells, and it lets you burn a Turn Undead usage to attempt a counterspell as an immediate action. Obviously valuable.

In general, the cloistered cleric variant is a great 1st level to take for a lot of people: you get 4×(6+Int) skills, much better than you would as a warlock, you get all of the above, another domain, Knowledge Devotion from Complete Champion, and you get identify as a 1st-level divine spell, i.e. you don’t need the arcane material component for it. The Undeath domain can get you Extra Turning if you want it, or you can use the other domain to get all manner of excellent things. Check out the Dipping Cleric Handbook for more ideas.

Divine Defiance requires “divine caster level 3rd,” which warlock doesn’t qualify for. The solution to that is Practiced Spellcaster (cleric) from Complete Mage; that will get your cleric caster level (but not any other aspect of cleric spellcasting) up to 5th to meet the requirement. You will also most likely want Practiced Spellcaster (warlock) to get back the CL lost when dipping cleric, as that would otherwise hurt your dispel checks.

The eldritch disciple prestige class from Complete Mage could progress your cleric spellcasting along with your warlock invocations, but it’s not that great and it would require you to take another two levels of cleric to get the 2nd-level divine spells it requires. Not worth it. Shenanigans can get you in without those levels of cleric, and that might be worth it, but if you want to go that route I recommend just asking your DM to adjust the class’s requirements to begin with, rather than get into shenanigans. For example, eldritch disciple expects a Cleric 3/Warlock 1 entry, and then at 1st level gives you invocations only (so you are effectively a Cleric 3/Warlock 2 at 5th level) and from then on progresses both. Maybe your DM would be willing to flip it, allowing you to enter as a Cleric 1/Warlock 3 and then progressing spells only at 1st (so you are effectively Cleric 2/Warlock 3).

The other prestige class worth mentioning for any warlock is chameleon from Races of Destiny. The second-level bonus feat can be changed every day, which means it can be any invocation you need that day (by making it Extra Invocation), or if you get far enough, an Item Creation feat to go with your Imbue Item class feature. This makes Cloistered Cleric 1/Warlock 9/Chameleon 2 a pretty strong build for you, and when you hit Cleric 1/Warlock 12/Chameleon 2 at 15th, you’ll be exceptionally versatile.

I will mention, however, that I have done a counterspelling warlock quite similar to the one I describe above, and while it works more or less, it’s pretty boring. If you are really interested in a high-power spin on the idea of ruining other casters’ day, what you really want is a Black Tactica build: use the war weaver prestige class to buff all of your allies as a move action at the start of combat, and then spend your actions in combat readying to target anyone who starts casting a spell. You don’t ready a counterspell, however: you ready the most reliable single-target nuke you have. In addition to hitting the spelcaster hard, which is painful, this also forces a Concentration check with DC 10 + the damage dealt. It’s trivial to pump the damage well beyond what anyone can hope to hit with the skill check, which means they will lose the spell.

This is a very high-power build that may actually make your game less fun, as your enemies start using similar tactics and combat becomes something of a deadlock, everyone readying nukes against the others, and whoever dares to move first still not getting anything done.