For reference on beams try

http://www.aluminiumwarehouse.co.uk/index.html gobsmackingly cheap tube and it is delivered the next day.

and for carbon tubes try http://carbonfibretubes.co.uk/ some of the big boat spinnaker poles are just the right size for us and its surprising the low cost on smaller tubes for tiller sticks etc, talk to them they always seem to have a bit laying around from a production run somewhere.

Why didn't I use carbon beams on Bitsa? Carbon is a great product for tensile strength, it is terribly weak and much inferior to aluminim if hit with something sharp or given the usual sailing abuse. To protect it you must use a reinforcing glass or kevlar outer layer which brings the weight right up to that of similar Aluminium tube. Aluminium is a great product, it just doesn't have the kudos of carbon fibre.