Backyard Aquaponics
A place to discuss aquaponics

I am looking for US vendors for 800-1200 gallon above ground tanks, not having much luck finding them.

They are out there but they tend to be a bit costly.

What sort of shape do you need? And have you thought about doing something on the order of a mesh tank with a potable water tank liner? Using Cattle panels from tractor supply to make the sides of the tank the diameter you want and then order a custom liner for the size tank you made.
I think I'll do that the next fence panel tank I make since the folds in the liner in my current tank kinda annoy me though it is working fine.

However, aquatic eco systems sells some tanks in that range and I know there are plastics manufacturers that sell rain water tanks that could be cut down. Then there is always the option of getting the fish safe line above ground swimming pool type tanks but they usually run a bit larger than you mention.

Plastic Mart has a wide range. Lots of items on sale and frequent blemished (or just overstocked) items at greatly reduced price.

In planning the next phase (add above-ground fish tank and turn my pond into the sump), I am weighing the pros and cons of all-plastic, mesh+liner as TCLynx has demonstrated, and just building my own ferrocement tank to my own specs and sealing it. You don't need a lot of ferro OR cement (or assistance) until you start going beyond 1000 gallons.

Rick