Anyone know where to find custom aquarium backgrounds?

You can select backdrop in websites like this one:

www.nanobioshop.com

They have quality wall aquariums and they offer various translucent background.

You can also request for custom background from them.

Custom Fish Tank Background

I would have to say it is a bad idea to put anything questionable in a tank. It is a closed environment and small amounts of contaminants can reek havoc with the biological system. I have set up many aquariums and putting in the type of rock or driftwood and even vegetation can make for an unhappy experience for both you and the occupants of the aquarium. Your idea of creating and putting formations behind the tank a great idea. If you could match the fore ground rock with forms you put behind the tank on the backside would create a greater depth of view. If it is rectangular shape tank, you could try and putting it in a corner at an angle creating a enclosed space the shape of a triangle behind the tank. There is a lot of creative ideas that space can be used. Back lighting makes for some nice effects. The options are yours have fun and good luck on creating the look you want.

It doesn't have to be an official aquarium background. Just get a Star Wars poster and tape it to the back of the tank, so that you see it through the glass when you look into the tank.

If you want to make it waterproof and extra-long-lasting, get it plastic-laminated.

You could just throw a collage together in Photoshop, size it to the dimensions of the tank, and get it printed at Kinko's and tape it to the back if you wanted.

You could paint one