Since you location is not shown on your profile, I don't know where you are, so it is that much harder to suggest a source of cheap leather. Most leather is not cheap, and assuming that there is no leather supplier near you (some have great bargain bins), you might practice by making smaller items. Cutting, stitching, dying, burnishing, etc. can be practiced without spending too much time on each piece. I would scrap the idea of purchasing the foam sheets, as they have nothing to do with leather. At the worst, practice with canvas that you can purchase at a fabric store. In fact, many products are made using canvas and leather, and they can look pretty good together.
That my 2¢.