How to protect yourself:
1) Do a search for the town/city in MX you're going to. Look for farmacias or graneros being recommended or look for the names of guys who know that place well---a lot of the guys here will be more than willing to help you if you send them an eMail with a polite request.
2) Never go down there without specifically knowing what you're going to buy. Better yet, look up lot #'s, pics, descriptions, ect. of what you are going to be looking for, and if its your 1st time, print them off & carry with you (be sure to throw your "literature" away before crossing back over).
3) Don't act scared or whisper what it is you want, they can tell if you're new to this, and if they make a habit of passing fakes to tourists, you'll be a primary candidate.
4) Never buy something you're not comfortable with. Usually, within 100 ft there will be another farmacia.
5) Try to buy as much as you can in the graneros. Go to
www.ttokkyo.com.mx and click on Distributors to find a list.