Over here in the UK, Exo Terra make a fake rock background to stick on the back of their glass vivs, so they do come in all sorts of different sizes but it's also made of Polystyrene so it's quite easy to trim down with a crafting or stanley knife to get the size you need. Exo Terra : Background / Rock Terrarium Background

If I remember right, it's a US company as well, so you may be able to find somewhere local to you that sells them.