McMaster-Carr offers lengths of 80/20's 1010 extrusion which is commonly used as a launch rail for rockets. They also offer 1515 extrusion which is used as launch rails for larger rockets.
A six-foot 1010 extrusion (part number 60585K41) is about $25. An eight-foot section (part number 60585K61) is about $30. Longer lengths are available, but shipping may be much more expensive for longer lengths.