Mitch, yes, ebay is the best place to find them. Any of the MB Quart are better than most, but I would stay with the Reference, Premium or if your loaded the Q-series. When searching, make sure you are searching for "MB Quart".
You could also try the JL Audio TR line, that is what I use for my tower system, and will be converting the boat over this spring. They are bacisally in the middle of the MBQz and the Clarion's. Crossed over right, the JL's sound great! KG