I know a good source, but it's a local place.
Try your wedding store and your local printing place. My favorite place does nifty printed invitations for parties and the local Bridal Store also orders napkins and such for weddings and showers. Try those places.
Linda C

If you can't ind anything locally, Google "American Stationery". It's a company in Indiana that sells monogrammed stationery as well as napkins and other things. Not cheap, tho.

Have you considered getting cloth napkins? I suspect you could get a few dozen that would last forever for the price of a few hundred paper that would only last a few parties.

Failing that, if you are just looking for a single letter, you may be able to find some pre-fab ones at a large party supply store. (plumparty.com has some cute initial cocktail napkins but the color scheme may be limiting)

Or if you are up for a little DIY, you could get an embosser and stamp your monogram onto any paper napkin you choose.