Where can I get a 3ft run to add on to my current run?

I have a spare 3ft as you do, and I want my little lad to have it. Does anyone know where I can buy a 3 ft run to add on to my 4ft run which I already have?

As I know 3ft isn big enough on its own but I want it as an extension.

Any help welcome.

Cheers

how about makeing an extension for it? x