well, you can't really buy "fiberglass"... the term "fiberglass" refers to a finished product (made from resin embedded with glass fibers).
I assume you're looking to buy the necessary supplies used to make a fiberglass item (or items). there are places to buy it online but it will cost you alot of money to have it shipped. fiberglass cloth or mat is cheap to ship, but polyester/epoxy resins, catalysts and cleaners are all hazardous materials which will carry an additional shipping fee/surcharge ($20-$60 [or more] per package, depending on the shipping service/carrier).
how much do you need? you may be better off just going to HomeDepot, Lowes, boat stores, etc. you may pay a little more for the product itself, but you'll save alot in shipping charges (if they have enough in stock, ask them if you can place a special/bulk order).