Hi there. I understand how you feel. I am 23 and got Cash just after my 21st birthday. I am also an animal lover and my parents and friends think I am slightly crazy. I now have two yorkies (June is 13 weeks old tomorrow). I wouldn't change anything!
Never buy from a pet store or a backyard breeder. You will simply pay more in the long run - on vet bills, surgeries, etc. Find a reputable breeder who understands your situation. They may be able to let you pay off the puppy over a period of time.
I hope you don't mind me asking, but what are your plans for the future? You need to realize that yorkies live for about 15 years. You need to think where you will be during that time and if you move out on your own or start studying or working, what will you do with the dog?
Simply because you aren't working or studying now, doesn't mean that you never will and you need to take this into consideration. I know that it is hard to want a yorkie so badly, but you need to make the best choice for yourself and your future dog.
Also, who will pay for the food, accessories, vet bills, etc? Your parents aren't going to support you for the next 15 years and having a dog while studying is really difficult (believe me, I've been there). It isn't impossible, but it is really hard. I have to scrape by on a meager allowance and had to start working part-time too.
Next year, I am starting to work and finding an apartment with a garden (or just a place where they allow pets) is very difficult.
So, ask yourself: What are your plans for the future? How are you going to finance your dog? Will you be able to afford emergency vet care? What will you do with the dog once you move out of your parents' home?
This is a huge decision to make and you need to look at it from all angles. Otherwise, your dog may end up in a shelter and you don't want that.
Once you decide that you are, in fact, ready, buy from a reputable breeder!
Good luck and welcome to YT!! Just by being on this forum, you have already done the best thing for your future yorkie!
