GS foam can be purchased in any hardware store.

It depends on what type of background you're looking to create. There are several options. Any ideas of what type of style you'd like to create? Waterfall? Wall of plants? Perches? Rockwall?

GS works well, as does styrofoam for cutting shapes. You can use both, or cut into GS after it dries to create shapes, shelves, plant holders, waterfalls, water-bowl holders, vines, etc. You can use 100% aquarium silicone to "glue" things together and/or to the walls. You can use silicone over the GS and apply substrate. You can grout over top of it. So many options .

It also depends on what you'll be housing in the tank and what type of needs they have.

I have yet to convert a tank, so I'm not much help there.

There was a site I really liked for making waterfalls. I'll see if I can find it.

Hope this helps .