I second ebay. I usually use ebay or etsy, but etsy is usually pricey. I have them create a custom graphic for me using my boys pics, then I order photo cards online at Costco or Sam's Club to pick up at their photo department. Usually $10 for the design fee on Ebay.
I ordered from Vista prints once, the invitation was more postcard like, if that is what you are looking winkflash.com has a special on cardstock photo cards from .23-.29 cents each depending on the size plus shipping.