Funny you should ask: I just received an order of both hinges and back strips from Foster & Smith today.

http://www.drsfostersmith.com/Product/Prod_Display.cfm?pcatid=13819&N=2004+113765

Glad to know that they sell them. The price is a little high though. You could buy a brand new onw for only $5-7 more for a 55G though. Well worth it for a 6' tank though. Glad this was asked and answered.
For a cheap fix though, try some hardware supplies for a plastic or acrylic continuous hinge. They are very inexpensive to buy single, but they are hard to find. I have seen them on the internet, but you have to buy in bulk. All you would need is some clear 2-part epoxy that is safe for potable water supply, about 44 at Home depot or hardware stores.

Woe;

I agree the price seems a bit high, but if you check the cost of a new, 4ft glass cover, it may be less, not to mention time and gas running around to try to find a match to the original, and paul did ask for easy along with cheap, so he will have to decide which solution best suits his time and wallet.

I really bought them to make custom covers, as I line up my 10's short end forward and want the covers to open on what would be the side, rather than front of the tank.

Clean it up and use like plumbers tape colored.Hubby has a grey,it s in blue,black.Put a piece on both sides waterproof.