I don't think you need to search very far -
See these quotes from the warlock class description (Complete Arcane P. 5-6)

WARLOCK

Born of a supernatural bloodline, a warlock seeks to master the perilous magic that suffuses his soul [...]
[...]
Adventures: Many warlocks are champions of dark and chaotic powers. Long ago, they (or in some cases, their ancestors) forged grim pacts with dangerous extraplanar powers, trading portions of their souls in exchange for supernatural power. While many warlocks have turned away from evil [...] they are still chained by the old packs through which they acquired their powers [...]
[...]
Background: Warlocks are born, not made. Some are descendants of people who trafficked with demons and devils long ago. Some seek out the dark powers as youths, [...] but a few blameless individuals are simply marked out by the supernatural forces as conduits and tools.
The exact nature of the warlock's origin is up to the player to decide;
[...]
In fact, many warlocks are created by nonevil powers - wild or fey forces that can be every bit as dangerous as demons or devils.

^{(All emphasis mine)}

AFIK, This is the source for the definition of warlock class in D&D 3.5, with Complete Mage expanding on it, including providing rules for warlock of non-fiendish origin, among other things.
At any rate, three things are stated here which are relevant to your question:

Not all warlocks have actively made a pact with a supernatural power

The exact nature of the warlock's origin is up to the player to decide.

The description provides three alternatives:

1. Your warlock made a pact with a "supernatural power" himself.
2. Your warlock's ancestors made the pact.
3. Your warlock is one of the "[few blameless individuals who] are simply marked out by the supernatural forces". He didn't choose it - they've chosen him...

So, if your warlock's is a "type 2" or "type 3" - he didn't make the pact himself.

Not all warlocks have a fiendish origin

In fact, many warlocks are created by nonevil powers - wild or fey forces that can be every bit as dangerous as demons or devils.

So, your warlock may have an elemental lord, powerful fey or even a slaadi or celestial as the source behind his power.

Every warlock owes his powers to some extraplanar or supernatural source creature

Even if your warlord didn't make a pact himself, and even if his source is not fiendish, there's no such thing as a "self empowered" warlock.

While many warlocks have turned away from evil [...] they are still chained by the old packs through which they acquired their powers.

While this have very little effect from a mechanical perspective, the warlock class is defined with this "built-in" narrative conflict or impediment - he has a would-be master - Some powerful extraplanar entity which have plans or invested interest in the warlock. Since this entity isn't as mighty as a deity, the warlock doesn't lose his powers if he deviates from his would-be master's plans, and he may even defy him outright. But, if you play a warlock, it comes as a given that you should expect some supernatural meddling in your affairs - maybe the warlock is merely observed, maybe he is not that important to that entity, and maybe it'll notice him only after he attracts its attention (by going with / against its interests or by simply becoming powerful enough to serve it as a useful tool).

So, while you are technically correct in that not all warlocks personally made a pact with a fiend, you should still work with your DM to define the source of your powers. At the very least, provide the type of that source⁽¹⁾, i.e. whether your warlock powers are fiendish / elemental / fey / celestial / etc in nature. But I'd recommend that you describe the source as an entity, with schemes and goals of its own⁽²⁾ - you'll be missing out on role-play opportunities, plot-hooks and character depth if you settle on a generic "its in my bloodline" origin.

Finally

As a side note, you state in the question that this group has a certain way of doing things, so I wonder how constructive can "smacking them with a sourcebook" be. Clearly, a healthy gaming group should be open to debate regarding rules, and perhaps even settings and campaign world elements (to some degree), but in the end, the DM calls the shots, especially when it comes to the narrative setting. So unless you can convince your DM and group to go along with your interpretation, you may have to play with them on their terms, or leave that group - no matter how many sourcebooks and splatbooks are on your side...

(1) I don't know how much leeway a DM has when running NWN2, but at a tabletop game there could be a vast difference in the way an invocation is described, and how NPCs react to such manifestations, depending on the warlock's origin - for example, a good cleric may instinctively oppose anyone who displays demonic powers, while being indifferent to someone using the same powers flavored as fey or elemental.
(2) The specifics of these schemes probably could be left to the DM to use/abuse without sharing the details and ruining the surprise for you...

They're there for you, the DM, to create something.

The usual complaint about the Forgotten Realms is that too much is already written, making it hard or impossible to learn the setting for DMs, let alone the poor players, or for DMs to fit in anything of their own.

Having "blank lands" like this scattered around the setting leaves room for DMs' own creativity. If you have an idea, but can't fit it into one of the more popular playing regions because the lore and canon conflict with what you want to create, these blank lands are there to give your idea a home.

As a point of history, Sembia (south of the Dalelands) was a large blank area right in the middle of one of the most heavily-developed regions in the Realms, in the original AD&D 1e Forgotten Realms Campaign Set. It was set aside for DMs so that they could place their own kingdom (or expand a home game into a Forgotten Realms game by locating their existing campaign kingdom there) without having to settle for a region far away from the majority of Forgotten Realms setting material.

TSR promised to never develop Sembia so that DMs' creations would not be interefered with. That lasted all of six years before Sembia was written up in the AD&D 2e Forgotten Realms Campaign Setting, and a bitter hue and cry was the result.

So thank WotC for leaving you these blank areas. They gave you a name and an evocative description to set fire to your imagination — now follow your dreams and create something awesome that your players will love.