McMaster, Grainger, or any real auto parts or machine shop supply. You're probably looking for neoprene or teflon sheet, not just "rubber". And if you really want to get slick...check out www.emachineshop.com where you can download CAD software, lay out your gaskets, and have them cut and mailed back to you all in one shot. (Although, I suspect it will cost more than $3 each.)
If the ready made gaskets are only $3 I would go with that. Doubt you could save much if anything trying to make your own. Plus how much time would you spend and would they fit and seal as well after you were done?
The water is always bluer on the other side of the ocean.