As you may know if you have looked anywhere else it is pretty hard to find any information about Sanshin. If you are interested in buying a Sanshin get an inexpensive one to start with. Don't pay more than $300 for your first one also keep in mind that artificial snake skin is okay. Some people say that real snake skin has a better sound but I have played Sanshin's with real and artificial skin and they sound very similar. One great advantage of artificial skin is that doesn't break.
I have bought every single one of my Sanshin's from ebay. I find that the pricing is very reasonable especially for the quality of the Sanshin's that I have seen. If you don't have an ebay account you can register for one within a few minutes through the ebay link directly below.
If you already have an account then you can just start looking around. The ebay search bar below is displaying what I feel are some good quality Sanshin's. Sometimes it helps to search for the term Okinawa Shamisen instead of Sanshin. For some reason I have found that using the term Okinawa Sanshin returns better results, but you may as well try both.