Reincarnated souls (originally from Mysteries of the Multiverse)
I am not a strong proponent of reincarnation but being agnostic, it makes about as much sense to me as our other forms of religion.
I don't see availability of souls as a problem. Depending on the school of reincarnation you subscribe to, there are all the animal and insect souls making their way up the ladder. If you prefer, I see no reason that there would not also be a mix of new souls along with those being transferred.
The idea that souls may flying around the galaxy looking for a spot to land is not impossible but I like to think of the process as obeying certain laws of physics and my personal favorite would be the new soul, old soul format.