With all the clip-art that's available now days, I would imagine it might be easy to design your own?
(I should try it out myself as well.)
Writing a letter is really an artistic process.
I love picking out a paper design with just the right mood or decor that might make the person smile, choosing ink colors that will complement the paper and motifs, selecting a stamp that goes with the theme, and, if I have time (and any creative gumption left), decorating the envelope.
A letter to someone is really a quiet gift of someone's thoughts, personality, and time.