I don't know if you have been listening to CNN today, but they are running a feature story about puppies coming across the border from Mexico. In particular they are speaking about the breeds of what they are calling "designer dogs" which are particular mixed breeds (kinda like Yorkie-Poos but I did not hear anything about Yorkies).
Apparently the pups are only about FOUR weeks old when they are brought to the US, and people are buying them in parking lots and at yard sales for $300-400!! And of course they are having a ton of health problems, poor babies.
The US has gone so far as to set up a "puppy patrol" to try to bust these freaks. They found TWENTY puppies in a car stereo speaker box for goodness sake!!
So take care!! Be very careful who you buy your pets from!