If you absolutely must have these creatures in combat and only in combat, then your only option is to turn them into figurines of wondrous power like the Bronze Griffin (level 19) and the bloodstone spider (level 11) Note well, your blade spider has errata applied to it, so it may not be as useful as you think it is. They exist at every level (therefore, have them priced at the level of the creatures) with the text:
"Power (Conjuration) Daily (Standard Action)
Use this figurine to conjure a [creature] (see below for statistics). As a free action, you can spend a healing surge when activating this item to give the creature temporary hit points equal to your healing surge value."
Just note that their hitpoints should be significantly lower than a "real" creature's, to reflect their eternal nature.
The reduce beast ritual (level 7, Dragon 405) is almost what you're looking for, but as the beast takes a few turns to grow, it's not appropriate for in-combat use. It is reasonable to house rule a paragon tier version of it that works with literal pokeballs. Note that most rituals are designed to not be combat appropriate, so make sure there's a significant cost involved with this.
Beyond that, I'd recommend rituals for swapping things out before you go into a dungeon.
There are many options that exist for Arcane familiars, like the Safe Retreat (level 15), unfortunately, they don't work particularly well for more... "real" things.
I would use the Primal Grove, not least because it's one of my favourite rituals for moving armies around.
The primal grove ritual (Level 19, primal power) creates a persistent extradimensional space:
A grove generates enough food and drink to sustain its occupants and has a moderate, unchanging climate. If you move objects into a grove, they remain there when you leave,
So it's a great place to keep people. With 10 minutes (or fewer if you specialise) and 4k (which at the levels you're talking about is literally pocket change). The 10 minute casting time is handy because you can't pull it out during battle, which means it doesn't need to be balanced for battle.
Alternatively, if you read the hireling rules liberally, it seems to be viable to have a level-appropriate hireling (cost * 3 equivalent to sage) be able to cast specific rituals.
What you'll need, therefore, are 3 pairs of sending stones (level 11) and 2 ritualist casters. The follow the voice ritual (level 16) allows the caster to teleport to the location of the last activated stone:
You fold yourself and each ally you choose who is within 3 squares of you through a sending stone you hold.
The real trick is that there's a 24 hour use limit on pairs of stones and casters.
Therefore, there are two operations that you'll be performing: "sending" and "receiving" The sending operation has one pair speak to you through one of your personal stones, the first one casts follow the voice, teleports himself, his buddy, and your ally to you. They drop off the ally, collect whomever needs collecting, and the casting buddy returns.
The third pair of stones is for your fellow PCs. Sometimes, you'll want to teleport an injured companion back, and you'll need to home in on a PC's stone instead.
This has been addressed in the latest Sage Advice.
Thank you, @V2Blast
Sage Advice
Does the Thief’s Use Magic Device feature allow them to use spell scrolls?
Yes. The intent is that a Thief can use spell scrolls with Use Magic
Device.
DMG page 141 is also of some help here (Casting Spells):
The ability check is still required to cast from the scroll but, as a Rogue has no spell-casting ability score, their's is always treated as zero when casting spells from magic items. It would stand to reason that a check would be in order to cast the spell from a scroll as well, the only issue is that it is just a d20 roll for them.
If you don't have a spellcasting ability—perhaps you're a rogue with
the Use Magic Device feature—your spellcasting ability modifier is +0
for the item...
It does go on to say that your proficiency bonus (PB) does apply but this is specifically called out as an ability check so your PB does not apply to those. If it were a Wand or Staff, is what I believe the PB is intended to use.
Best Answer
Speaking stones
Speaking stones (WGtE, p. 44) allow for repeated long-distance communication. The stones allow for the holder to send a short message to any other speaking stone. [H/T Ryan Thompson]
Galder's speedy courier
The Galder's speedy courier spell (LLoK, p. 57) allows for instantaneous long-distance transport of small items. Adding the ability to cast Courier to the Speaking Stones from above should allow for limited transport of the required items. Longer or more detailed messages can be written and included in the courier.
Modifying the restrictions of the above to allow for use at the desired frequency and carrying capacity and you should have a (mostly RAW) network of devices that fulfills your needs and assists your players.