What aimless said! I haven't used a "footprint" in years and have had no problems, even with lightweight silnylon floors and a fairly large dog.

EDIT, LATER: I did, in the dim distant past, use a piece of painters dropcloth cut to fit. However, I gave that up about 1995. Yes, it's basically disposable, but with a duct tape patch or two, one did last for a 9 day trip.

If your heart is set on a footprint, check out polycro. This is the shrink-to-fit stuff used in storm window and door kits. It's also sold separately (but more expensively) by Gossamer Gear and Mountain Laurel Designs, among others. It's about the same weight as that painters dropcloth, but far more durable. Many tarpers use them for ground sheets, (you do need one with a tarp) and some claim they've lasted a whole season. With polycro, cut it a trifle larger than you need, because it will shrink, especially in warm sun.

You undoubtedly know this, but for any beginners who might be reading this--the footprint, if you use one, should be about 2 inches smaller each side than your tent. Otherwise it will catch trips off the tent side and funnel them underneath--on top of the plastic!