AP is your best bet. I just put together one of their 32qt economy racks. They mention in the directions about the possibility of separate heat tape for each shelf, but you do have to get the heat tape yourself if you go that way because they recommend snaking one piece and only sell it that way or no heat. Dragons4u used to sell ABS racks with slots for heat tape that you supply. Lately they only advertise their new PVC racks that come pre-wired. I think they do still make the old racks, but these are made to order like AP and their wait for made to order items is about the same as AP.
One consideration about the heating empty shelves. To get a Herpstat to power 4 separate devices, you'll have to get the Herpstat 4. The last I looked it's $340. As low as the wattage is on heat tape, I don't think you'd ever recoup the extra money for the t-stat.