If you look up next to the 'New Topic' button of the forum, there's a search box. You can search for most things before starting a new topic. Questions about litter and such have been covered before, so you could probably find the answers by searching. I would suggest searching "litter" in the general questions forum (this one) and the housing & accessories forum.
I wouldn't reccommend cardboard. I was using a cardboard box lid for a while because it was the right size, but trust me you will need to wash out the litter box with soap and water and the cardboard will turn to mush. I'd reccommend investing in a plastic litter box. I'm currently using plastic serving trays I found in the "picnic" section of superstore for under my hedgies' wheels + plastic rabbit/ferret corner litter boxes for the corner opposite to the wheel. My hedgies' pee/poop on one or the other, usually both and they clean really easily.