I just won a turboexpress and its in great condition except that its missing the BACK BATTERY COVER. I won it for $56 shipped on ebay but I was wondering if I should go ahead with it because I seller said I don't have to buy it if I don't need too (I guess he wanted more or someone else is offering more money) but how much would it cost to get another battery cover?
That's a great deal, even without the back cover. Keep an eye out on ebay for a parts machine. You can buy it for the back cover, and then resell it again to someone who wants one for spare parts (assuming the price is right). Sometimes they are pretty cheap.
As for power, I personally use 6 NiMH AA cells. They last a very long time in the Turbo Express, on a full charge.
You should be able to get away with any AC adapter (converts to DC) that has one of those selectable voltage switches on the device. As long as it provides about 7 V DC (or something close to that, batteries are never so precise, as my AA cells are only 1.2 V on average each) you should be fine. Just make sure that the polarity is positive in the center of the plug. You can usually swap those pins to swap the polarity. Look at the small picture on the Turbo Express DC power plug.
Take it into Rat Shack and some nice cellphone salesman (sarcasm) may be able to help you find one.