Hmm... you mean, substitutes for sticking lego people on to stay on, or just flat, hard surfaces? For the former just use a lot of smaller plates and connect them together (This only works for small battlefields unless you have a LOT of plates.). For the latter, you can use the table, the floor, the shelf, THE DESK, or if you have carpet in the room, a bunch of thin hardback oversize books places together.
I've personally never had a problem with this, because my Lego collection being as huge as it is, has a huge supply of baseplates. I have my main base, a jungle, an airport, and then there's my brother's base, all pretty darn big and built on baseplates, and we still have some left over.
For a nice, awesome battlefield, I really recommend buying some baseplates. Bricklink.com has some baseplates cheap, just look at this link (http://www.bricklink.com/store.asp?p=pd ... D=10880997
) for example, this guy's got 6 32x16's for 2 bucks each. I've personally never bought from Lego.com, everything is SO very much cheaper on Bricklink, you can get any part you want for rock-bottom price.
I really recommend biulding a battlefield, it adds a LOT to your game. A game in a beatufully designed jungle, a grey, lifeless military base, or a thriving city street is always better than, say, on top of a book on the floor. If you've got baseplates and bricks, the two most basic things, you've already got a great battlefield. Just connect the baseplate, and build some houses with the bricks. You can be as decorative or as bland as you want, but the more things there are on the battlefield the more things there are that the minifigs can interact with. It's so very much better than stereotop games, if you ask me.