A traditional deck type pizza oven operates about 500-600 degrees. I have been in the pizza business for over 25 years, and have worked on several types of pizza ovens, and they are all very simple (even the new conveyor ovens).
Basically the box is made of steel angle iron or square tubing, with sheet metal inside and out, with some good insulation in between. If you were closer to me, I could give you some insulation, as I have to scrap out 2 electric ovens right now (although it probably wouldn't be enough to build an oven of the size you need).
The gas pizza ovens use a basic gas valve, a safety valve, pilot assembly, and a temperature control. The burner is basically steel pipe, the only odd part is the casting that holds the orfice, and air adjustment.
If I remember right, a Bakers Pride Y-600 oven produces about 180,000 BTU's. Old bakers Prides can be found at reasonable prices from time to time, but usually bring higher prices because they are such great ovens. They have a "stone" floor in them that is about 1" thick, and transfers the heat to the pizza very rapidly, causing it to rise quickly, and giving the pizza a great taste. Other brands such as Blodgett usually sell for little at auctions, as they don't make as good of a pizza (I don't think they have the same "stone" type floor), and could be a cheap source of a burner assembly.
I have also seen large rotary ovens and bakery ovens sell cheaply at auction, and they would probably be large enough for what you want to do. The rotary oven would need some modificatuion, but the the bakery oven would probably work as is. Some are large enough to wheel in several racks of bread or rolls at the same time.
Watch for bakery or grocery store auctions (many have bakerys). I don't know of any companies that sell that type of stuff in your area.
Here is a company, that sells a lot of WalMarts and Sams clubs around the country:
http://www.mjauctions.com/
Here is one that works mostly in the mid-west, but also does auctions in other parts of the country:
http://www.grafeauction.com/
I have seen some of these large ovens sell for scrap prices because they are too much trouble to take apart, move, and store, for the average equipment buyer to mess with them. Unless they have a buyer lined up, they would rather buy smaller equipment that they can turn over quickly. Large bakery ovens usually aren't in large demand.