Speaking of post cards, we had ours printed at www.clubflyers.com. They'll print 5,000 post cards with full-color on both sides for under $200. That's a great deal.
A lot of online printing companies will offer one product at ridiculously low prices as a loss leader, hoping you'll like the quality and service so much that you'll buy other products, like business cards, brochures, etc. from them. You could do that, or you could shop around and find the printers that use business cards, etc., instead of post cards as loss leaders and get great pricing on those products.
Here's a list of just a few online printing companies I know about that specialize in small orders. I can't speak specifically about all these companies, except to say that people I know have used them for various projects and been satisfied with their work: