We have crabs that can live in freshwater in Australia, however they are predators that eat fish and plants so are not recommended for community aquariums.
Your best bet, depending on where you live, is to head down to a local creek with a crab net lined with flywire or mosquito netting (available from places that sell material for making clothes). Then bait the trap with a bit of meat and drop the trap in and wait. Do it at night and see if there are any small crabs that crawl onto it.
Alternatively set up a salt water aquarium and keep marine crabs. There are lots of really cool little marine crabs and you can have rocks decorations and crabs living in there.