the one at Wal Mart should be fine, I saw it too AFTER I payed 25 for mine at Lowes... it doesn't look a bit different than the one I have.
Basically a water trap is just a cylinder with a drain in the bottom to let the water out... if it is a filter/moisture trap then it also has a filter like a pumice stone almost, that keeps larger particles from making it past the water trap to the airbrush hose...
good luck!