I currently have a 150 gal. standard shape tank.  I am looking for a 210+ gal tank.  The issue is that the cost is exponentally higher once you go above 200 gal. I am having trouble finding a used tank as well.

I am considering building one but I wondered if anyone has any experiance with this? 

Or any info on were to find a large tank that isn't so expensive. 

It is hard to find ones that large. I'd say craigslist or a local aquarium club.

As this is in the breeding/prop section, whats the purpose of the tank? Breeding of clowns and such is better accomplished in smaller tanks. Corals in shallow tanks. Larger tanks like you are talking about are more for display.

If you could elaborate on the use of the tank, I might be able to help with some specifics.