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You can ge them at the dealer, but they're expensive as $hit. You might want to try Shokan, they have some decent used parts prices. It was already mentioned, but Ebay isn't bad.
I agree with the last post. You probably don't have to replace everything if you got a component speaker setup it should work just fine off the stock headunit and amps.
I don't think the Bose speakers are a 4OHM speaker, most after market speakers, execpt subs, are 4OHM. Bose amps are designed to work with the Bose speaker which if I can remember is 2OHM or lower.