In the US, Frank's Aquarium is probably the best bet. He seems to make an effort to get stuff properly identified, which simplifies your job when it comes to providing good conditions. Sachs Systems Aquaculture supplies quite a nice variety of native brackish water stuff to labs and hobbyists. I hasten to add that I haven't used either store, so this isn't an endorsement or recommendation. Caveat emptor!
I've bought from both Frank and Sach's, I can tell you they're both excellent sellers. Unfortunately, either has much in terms of brackish inverts at the moment.
Correct. Crabs generally aren't aquarium animals. Think of them more like frogs or terrapins: animals that need some land, some water. The one exception is the blue-legged hermit crab. This is widely sold as a marine algae-eater. Clibanarius tricolor is the species. It does very well at SG 1.010 upwards.