Does anyone know where I can get a powerful space telescope with a 20" mirror?

Answer 1

Here is a site that gives you several sources for 20" telescopes.
http://www.bigtelescopeguide.com/20-inch-telescope-mirrors.html

Answer 2

Here's a few:

http://www.obsessiontelescopes.com/
http://www.starmastertelescopes.com/
http://www.starstructuretelescopes.com/new_web_site_sst_002.htm
http://www.rcopticalsystems.com/