what type of oven is used heating plastics up to 800 degrees?

Look for a Grieve model #NT-800 heat treating oven. MSC lists it at over $7,000 which is out of your price range. I don't know what part of the country you are in so I can only suggest that you contact Grieve directly and maybe they have taken in a few trades which they would like to resell.

http://www.grievecorp.com/catalog/Batch-Ovens/Benc...

Here's one for $3500. Most of them are running close to $15K.

http://www.whkay.com/2758.html

http://www.whkay.com/

I don't think there's any common plastic that will handle temps like that, most become molten around 250-400 F. You'll have to check with an industrial supply company like Graingers or MSC. In reality if you had someone who could make a sheetmetal shell, it would be pretty easy to make your own oven for a couple hundred dollars with readily available parts at any good HVAC supply house.