A simple ,cheap footprint would be plastic sheeting, like painter's drop cloth, cut to the tent dimensions or slightly less
House wrap such as Tyvek is often used to make DIY tent footprints. The footprint should be slightly smaller (3 to 6 inches) all around than the floor of the tent although if your tent has vestibules you can extend the footprint into those areas keeping it smaller than the edge of the fly.
Optionally you can add eyelets to the corners and use some light bungee or cord to attach the footprint to the feet of the tent's poles or to corner stakes to keep it in position.
The smaller size is to ensure that rain and condensation shed by the tent isn't immediately collected by the footprint and accumulated under the tent floor where it will find any hole. If you want a groundcloth outside the tent use a seperate piece of material.
Great suggestion on tyvek. Would that be more durable than drop cloth plastics?
Tyvek is what I use. You can find it in Lowes or other stores. If your lucky and a new subdivision is going in near you, the builders have tons of it...