Some samples of good business stationery to use as "correspondence cards" for thank-you notes:

Crane's stationery is available in a lot of card shops. It's a little pricey but very appropriate. These initial cards run about $14.00 for a package of ten. http://www.crane.com/prdSell.aspx?SubDeptName=CranesCorrespondenceCards&Name=cc30z1_InitalCards

Another Crane's product, the bordered correspondence card. A little less pricey--these are about $15 for 25 cards and envelopes. Note the professional color stationery and the simple, elegant, border. Again, blue or blue-black ink is recommended.   http://www.crane.com/prdSell.aspx?Name=pc3716_EcruwhiteandRegentBlue

This is from the American Stationery Company, http://www.americanstationery.com/shopping/prod_detail/main.asp-pid-50-cat-21-site-1-sid-659391820C74FFD83442EF67. If I ordered these, I would go with the block printing style (e.g. Donald Prescott) and not the italic, wedding-invitation style. If you had these custom-made for yourself, they would run about $36 for 25 cards.