Plumbing supply stores. They will also have the correct solvent/glue for your application. Usually sold in 10' lengths. You can also get it at many hardware stores. A 10' piece is usually cheaper than a 3 ft' piece as you are not paying for the cutting (or "custom" sizing).
Thanks, I guess I'm going to plumb my 90gal with it rather than the white so it's not as noticeable. 1 1/2" from the tank and 3/4" back. Sound about right?