Looking for plants

Hey everyone, if anyone has any extra plants they don't know what to do with (or you don't want them)im willing to take some, since i have some spots in my 15 gallon tropical tank which need to be covered.
Im also looking for a fast growing plant which will cover the bottom of the tank.... have any ideas?
At the momment i can't pay since im saving up for a house in the summer.

Anyways thank you so much.