http://www.musikit.com/

http://www.larkinam.com/category.asp_Q_c_E_110

The book, Homegrown Music by Marc Bristol, is worth finding. It contains plans and instructions for building several diffent instruments. The instruments are simplified but fully functional. This is the book that got me started building my own. By building from scratch, you save money and have the wonderful feeling (and bragging rights) of having done it all yourself. Modding is eisier on a scratchbuilt instrument for a custom instrument.
Here's a link I found to one copy for sale. It's a book from the 70's, so may not be easy to locate at any given time. Check your local library as well.

http://www.abebooks.com/servlet/SearchResults?sts=t&y=6&kn=musical+instruments&tn=Homegrown+Music&x=81

Whichever way you go, building your own is fun and highly rewarding. Remember, it's just wood. Cutting, fitting and gluing is no harder than building a speaker box, just smaller. If you can cut a small drawn pattern and sand it and glue it, you can build a wooden instrument.

Dave