-lawn and leaf bags aka trash bags
-fabric/sheets/shirts for fat people
-spray paint
-construction paper/poster board
prettymuch anything and everything will work.. the less glossy the background the more the fish/plants will stand out.. the most important thing to remember is to get the back of the tank clean before you put the background on..
i use this black fine material cloth that's used when laying down gravel or lava rocks. it blocks weeds from growing up. it looks really nice because its very thin and has a marbled look to it. its also water proof and comes in huge rolled up sheets. so u could do backgrounds that are over 72'' long by 36'' high! in single piece of this stuff. its also very cheap! get 100ft by 36'' for under $10.
this is way cheaper then going to a fabric store..