Just want to let you know that while you might have your heart set on a younger dog (3 or below), Yorkies do tend to live quite a long time. I say this because I got Sandy through petfinder, she's about 8 years old and a wonderful companion. I didn't have to go through a lot of the hard puppy things with her and she ended up being probably the best dog I've had so far in my life. You would never know she hasn't been our dog her whole life.
I'd keep an open mind as far as age goes. You'd be surprised at what you can find. It took me a year of searching before I found Sandy. I had started out wanting a Yorkie puppy and ended up with an older mix. It all worked out perfectly for me, and I felt like I was doing a good thing for Sandy as well.
It's hard to find a Yorkie for under $500 dollars anywhere. You may have to explore other breeds or options if you have your heart set on a young female puppy.