What cool and affordable millipedes could I get as a beginner?

I originally got my tank for a leopard gecko but I realize I couldn't afford to keep care of it so I thought "hey I got isopods why not a millipede?" My mom suggests i should get a bigger and affordable millipede! I have a small heat pad on one side of the tank and a UVB light that was originally for a leopard gecko. And with your suggestions can you also tell me where to get them? Im from IA :D