TL;DR: How can a character produce, ->via spells<-, creatures that can be True Polymorphed into objects without disappearing later?
[NOTE: for the scope of the question, I am not concerned with the character's class or what spells are available to him. Please consider any spells published in D&D 5e books/supplements as acceptable ingredients for a solution.]
I am interested in a character being able to use True Polymorph to create arbitrarily many objects (max 1 per day, of course) to use as spell components, build his secret lair, etc. The catch is, for black-boxed reasons, he is sequestered away from the world, and can only use resources that have ultimately come from his own spells. So he can only T-P creatures out of creatures or objects that he has previously produced, and only T-P objects out of creatures he has previously produced.
Creating objects to use as fuel looks straightforward. Here's one repeatable example:
- Cast Demiplane, choosing "stone" as the material of the "empty room". We now have a stone surface.
- From inside said demiplane, cast Wall of Stone, creating ex nihilo a volume of stone "merging with and solidly supported by existing stone". By concentrating the full ten minutes, the stone wall can no longer be dispelled: it has become permanent, mundane stone.
- Cast Fabricate to shape objects of desired dimensions out of this stone.
Creating creatures, however, seems to be more complicated than I originally thought, since it seems that, by RAW, one cannot "stack" a second T-P onto a target which already has a (permanent) T-P effect on it. In other words, if I understand this correctly, creatures "created" via object-to-creature T-P are not actually valid targets for a subsequent creature-to-object T-P.
If T-P itself won't give me creatures to use as fuel, where can my character turn?
Conjuration? I've looked into spells that conjure or otherwise summon creatures, but nearly all of them have a limited duration, and (as linked in the "TL;DR") any objects T-Pd from them are liable to disappear after the summoning spell's duration has elapsed. The few that do not expire seem unsuitable for other reasons (Find Familiar/Steed seem to be manifest spirits, and are not called "creatures"; Planar Ally is not a creature of your creation, and would probably not take kindly to an attempted T-P).
Breeding? Suppose we T-Pd a bunch of stones into rats (PHB p.309) and bred them. There are plenty of spells that could provide food and water for them. We would not be able to use any of the initial, generation-0 rats for creature-to-object T-P, but what about the progeny? It seems reasonable to suppose that they could be, and it seems feasible for a character to maintain a cageful of rats, but there's a lot here going well beyond RAW and into the realm of DM fiat.
Is there any better, more explicitly RAW way to obtain creatures for this purpose, using only a character's own spells?