I do two different things. I print (on an Epson R300 using Costco photo paper) six notecards for every client, with gold lined envelopes that I get at Staples, as a thank you. If you pay someone else to produce them they typically want to print hundreds at a time to make their setup costs worthwhile. You could go to Kinko's and get color copies done on card stock that you could fold into cards, but I don't like their color and can get much better results myself.
I also use
www.modernpostcard.com to print 500 or 1000 folded notecards with one or two of my best portraits on them to use as notecards the rest of the time.