I'm not sure what the seal looks like that you need, but here are a couple of places I have bookmarked incase I ever need to replace any seals. I think the installation of seals should be pretty straight forward and might be something you could do your self.
http://www.all-rite.com/ Look under products/moldings/foam & rubber seals on this site.
http://www.rvsurplussalvage.com/cata...ategory_id=106 This link goes straight to the seals.
Hope this is helpful as $500 sounds like too much to me.