Occasions pop up all year long, especially after the holidays, when it's polite (and sometimes necessary) to send a Thank You Note. I like to have stationary and stamps on hand so there's no excuse to procrastinate. For most Thank You Notes, a Correspondence Card (heavy card stock, no fold) or Fold-Over Note (most formal) is appropriate. As for the note itself, make it heartfelt and sincere. If you don't have much to say, take size into account when choosing your stationary. Less space means a shorter note!
(it's perfect, thank you so much, i love it, wow, how did you know, you're the best)
Tiffany Dragonfly Correspondence Cards, for any kind of Thank You. Make it personal with Ensley Initial Notes from Sugar Paper. 
(personalize for an additional fee)
Make your own!
For our holiday thank you notes, I'm using wrapping paper (recycled from a gift to Pearl) to cover notecards and line envelopes. I followed this tutorial but used permanent double-sided tape instead of a glue stick and scissors instead of a paper cutter. For decorative paper, check out Paper Source. "Feeling gratitude and not expressing it is like wrapping a present and not giving it."
-William Arthur Ward
