It may help us to give you better information if you tell us why you can't buy diapers.
Is it that you don't have the money, you are afraid your parents/housemates/roommates will find out, there are no stores near you, you can't order on-line, etc.
As was suggested, you can use some towels, along with go to the store and buy some diaper pins, those are easy to hide and would help hold the towels up.
I feel like there is a story here. Perhaps some elaboration on your situation might help shed some light on how we can help you? Do you work? Is it money? Do you need advice on how to hide diapers? Help us out here.
When young I would cut a trash bag into an hourglass shape with a towel in it, then just use a bunch of tape to get it all in place/ tight enough that I won't leak.