Best I've used have come from the clearance section at Kroger (Graceland, actually, though it was 2 years ago). They're non-stick roasting racks. Turn them upside down and voila, rib racks.
Non-stick is a real winner. Compared to the cheap brinkmann or charbroil ones, they clean very easy. You don't have to spend time getting all the gunk off the inside.
Another thought is to look for cheap (i've seen them for around $10) roasting sets that include the non-stick rack and pan.