Treadle belts come (came) in about three different sizes. 3/16", 1/4", and 5/16". The skinny 3/16" is what most folks sell now and I think they are too skinny for anything serious. There is a person on this forum selling 1/4" ones but I have lost that info ... again.
They come in one basic length, 72" and you have to cut them to the right length.
A good place to order treadle belts is Sew-Classic:
http://www.shop.sew-classic.com/ . She also has a good set of instructions on how to cut them to length.
I suppose you could use Lexal leather conditioner on a good belt to keep it that way, but one that's already dried and cracked is pretty much toast.
Joe