For factory speakers, either Ebay or GMPartsdirect.com. GMPartsdirect has decent prices, but they kill you on the shipping.
The short answer for aftermarket speakers:
If you're going for function over form, you can just buy some coax speakers from Wally World wire them in and go. I think the factory speakers have little clip connectors on the back, so you might have to do a little wire splicing for the aftermarkets, no biggie.
The long answer:
The front door speakers on Firebirds are component speakers, so they're wired to separate highs/mids, so when you replace the midrange speaker only with a coax, you might not get a good, clear sound quality b/c it's only being fed midrange signals. Same goes for the rear speakers, as they are fed mid/low signals because they function as subwoofers. To do it right, you'd need to purchase component speakers ($$) for the front to replace both the tweeter and midrange (or rewire the factory stuff) and a standard midrange speaker for the rear (next to the seats). If you were really interested in doing it perfect, and had a lump of disposable income, you might as well spend the dough for a decent head unit that will run all 4 speakers and sound a lot better.
Just leave the Formula and some speaker parts with me, Toddles, and I'll make sure you get it back in a month of testing...testing the stereo...yeah...that's it...