Sorry, don't know any in bay area (bay area of where???)
I would try local office printing stores, they sometimes can print on apparel. If you can do the work yourself, I'd suggest getting printable iron on transfers. You can get a digital image of your drawing, print them out on your computer & iron them onto your t-shirts. However, 500 IS a lot, but if you're desperate, its the best way. I've done 20 of these before, it took some time, but it looks great.
If its for charity, there may be companies who will give you discounts in the printing. Google search T-shirt printing or apparel printing in your area.
if you want to do it online, (where they do the printing & shipping, etc.) try cafepress.com, they usually will work with charities & may give you a discount, though I do believe they're expensive no matter what.
Sorry can't help you any more. You may want to re-ask this question again, try a different category, such as: Society & Culture > Community Service
Games & Recreation > Hobbies & Crafts
one of the categories under: News & Events
You might have to get at LEAST a tax-id or a basic business S-Corp filled out with the government in order to setup a wholesale account. I'd find a tax guy/gal and talk to them about doing that first.