While your requirement is a bit wide for any heatsinks available from retail sources, I'd say you have several options.
The first thing you want to do is to get familiar with two types of hardware specialty stores found in most cities: an "electronics warehouse" that supplies parts to jobbers and individuals; and a sheet-metal supply store and warehouse. Both stores sell "hardware" or tools for those specialties -- for instance -- the sheet-metal establishment would have such things as hole saws, micrometers, pop-rivets, hack-saw blades and so on.
My "electronics warehouse" carries very flat (1/2" tall fins) aluminum heatsinks that are about 2.5" x 6." You're looking for a heatsink that is about 4.75" square. You could buy two of the heatsinks I've described for $1.50 each and cut pieces with a Dremel that would fit your spec. I would also check on-line for a store named "allelectronics.com." They sell peltier coolers with heatsinks originally manufactured for automotive beverage and food cooler applications. They probably also have an assortment of heatsinks.
Cutting the heatsinks I've described with a Dremel is a minor operation that shouldn't take much time. You'd seek to cut them between the fins in a straight line with a fiberglass reinforced cut-off wheel, or you can "groove" the metal with the Dremel, put it in a shop-vise and finish the job with a hacksaw fitted with a blade for sheet-metal.
You can also buy blocks of aluminum of various sizes at the sheet-metal supply. Here, it seems like too much trouble cutting the fins from such a chunk, although you might reduce the work involved if you use the Dremel to start the cuts (as already explained), and then proceed with the hacksaw. Any flat surfaces that need to be mated with the Peltier can be lapped for flatness.
If you can find discarded CPU heatsinks of the right size, you might be able to use two of them.
Whichever path you choose -- and there may be pre-manufactured heatsinks available to fit your size requirement, this doesn't seem like an insurmountable problem.
PS Make sure you use a pair of shop glasses and some sort of respirator if you find yourself cutting on aluminum with the Dremel. I'm not aware of the toxicity of environmental hazards of aluminum dust, but the metal is light, and the fine dust doesn't just fall out of the air like pieces of "mild" sheet-steel. I really don't think you want to breath it. If you're filing or cutting the metal with a saw, the particles are much bigger and don't become airborne, so you might dispense with the precautions at that stage.
