I doubt you will be able to purchase these, even from a biological supply company or a live food supplier for fish. I may be wrong, but I’d suggest you’d probably be better off collecting your own. Have you considered including terrestrial amphipods?
Collembola and terrestrial amphipods are found in sheltered, permanently moist places, such as underneath flat debris under houses and in deep decaying leaf litter. A good sized aquarium net and a bucket are all that is needed to collect them. Just put some moistened scrunched up paper or litter in the bucket to give them somewhere to hide and stay moist. Slaters/woodlice can be found in similar circumstances. They are also very common occupants of the base of potted plants with plenty coarse mix at the bottom, that are kept on the ground. I once counted 49 slaters sheltering in one 14cm pot containing a bromeliad that I was repotting.
You might also include earwigs that eat decaying vegetable matter, not the species that eat living plants. They are even less prone to desiccation than slaters. They can be readily collected from under bricks or wood or pots on the ground where wind blown leaf matter has built up alongside or where debris has been thrown over vegetation (as already mentioned by Smittiferous).
