For the novice lime putty can be prepared from hydrated lime found in any masonry supply yard as described in "How to Make Lime Putty" topic in this forum.
Direct link below.

http://www.truefresco.com/dcforum/DCForumID1/61.html

Look for sand in masonry supply yard also. Play sand from Home Depot can also be used (sift it through window screen for intonaco)

There is no specific fresco supply merchant in the US for the moment.
We are planning to offer some of the specific fresco materials through TrueFresco.com in the near future.

Thank you for the feedback!