Cardomain - very customer friendly and they'll price match any other authorized quote and even if the price you want to price match isn't authorized they'll still get they're price low enough to look very attractive.

Ebay - can be great if you know what to watch for.

Used Section - in the various car audio forums can be excellent if you know what to look for.

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Pioneer - MB Quart - OZ
http://www.cardomain.com/id/37554
"Your sister is hot but your mom does that thing with her tongue..."