try PepBoys or a place like that, they can probably order you one. If you're more in a hurry someplace that sells trailers would most likely have one or again, could get one. In Houston we have a place called Trailer, Wheel and Frame that does nation wide sales you could try looking them up.
Check the local autoparts stores. If you don't mind spending an extra $75 uhaul charges about that for installing one of their units. It's an easy install but I paid to get it done the 2nd time around.