I would try the pawn shops before I would go to the flea markets. There is a large pawn shop on the corner of "F" Street and Tyler in Harlingen. Another one near the Valley Morning Star - at the intersection of Commerce Street and 77 Sunshine Strip. There are several more around Harlingen. Check the yellowpages.
Over on Flickr there was a
thread that said Jano's Furniture on Harrison Street had a small selection of cameras. It didn't say if they carried used cameras, but it might be worth stopping by.
There is not much in La Feria. I hope this helps.
Rob