I use needlepoint canvas in my tanks, bent over so it's like a ramp and weighed down with a stone so it doesn't go anywhere and the crabs can't get underneath it. It's got excellent traction for both claws and feet, and the crabs can then get right up to the water's edge on the ramp, or they can still hang over the edge of the bowl and drink. (About half like to wade, the other half hang.) It's cheap, and normally pretty easy to find even at Walmart.
I'm currently using plastic trays from microwavable ramen as dishes as they're shallow-ish, but any type of food-safe plastic will work - just check the storage isle at the store if you don't have anything laying around!