try indioaudio on ebay, i bought my MB Quarts from them, and had a problem with one of my speakers about 6-months in, and they honored their one year warranty and hooked me up with a new pair.
anyway, i called maxxonics, the new mb quart owner, asking them where i can buy their stuff from authorized dealer
tey told me 2 local shops in Illinois
http://www.onlinecarstereo.com
http://www.sounddomain.com
mosdt of them do not even carry Premiums
so i will stick with ebay, or ikeaudio.com