I got the copper line at Breed and Company, a local hardware store. It
should be available at any decent hardware store.. thats pretty odd that
you are having trouble finding it.

I used 3/8" copper tubing. I doubt that 1/4" copper tube will flow
enough fuel to adequately power the engine.. so I would try to avoid anything less than 5/16 or 3/8,
especially if you are going to install this on a truck or bus.