I think Dice (kevin) sell's the aluminum extrusion from Hepco Slide Systems Ltd.

You should have told him your plan, he would have sent the sides you need with the first order im sure.

He has also got the extrusion listed on his website, measure your current gantry sides then match them to whats on his site ?

Drop him an email, dont think he has got the whole Hepco range on his site, watch out for the VAT to.

Cheers,