Personally, I think the choices for bases is limited only by your imagination. I normally use a plain piece of semi-finished oak shelving. Because I normally put down ground cover made from Celluclay, I make sure I seal the wood with a good sealer. I personally like Minwax's Acrylic Polyurethane. Sealers are required to keep the wood from warping. I would advise staying away from pine or other soft, light woods if ground cover application is going to occur. Wood from trophy plaques also make good bases, as does plexiglas or even a piece of an old mirror. I've even seen bases made from ceramic tile. I had some trees damaged during Hurricane Isabel this past fall, and had to cut them down. I sliced some thin pieces (approx. 1 inch or so) of trunk from both a pecan and a poplar (8-10 inch diameter) and set them aside to dry in my garage in hopes of using them for some small artillery bases in the near future.
Bottom line: the possibilities are endless. Experiment and find something that you like that really complements the kit you've built.
Gip Winecoff