Try Pet Edge. I'm fairly certain that's where I found both of mine. The best advice I can give is buy the best quality you to can. The finer toothed thinning shears will give you a smoother finish.
You also may want to have a look at YTs grooming library:
Grooming Library - YorkieTalk.com Forums - Yorkshire Terrier Community
Good luck & remember, it's just hair & will grow back if you mess up. Just aim for a little longer than you really want.
