Hi Rich,
the MSS system is not readily sold in component length. A number of companies sell the system in the UK but this is based on them buying the extrusions in bulk. Although the interlocking dovetail design is simple the actual process of producing the extrusion (and the variable tolerances as the aluminium cools down after stretching) means that a lot of them do not happily fit together and is often a case of choosing a good pair that do.(For the same reason a lot of scrap is also produced!). I guess you already have a supplier but if you need another please feel free to give us a shout. Hope this helps?
Andy