(the Metalbestos folks), offer a single-wall pipe system in stainless and AL29-4C, I believe. Selkirk is an umbrella company for Heat Fab and other system manufacturers so there is a lot to look at.
I do not think that you can buy directly on-line unless via one of their partner distributors.
Derheatdude is onto something. You really need to figure out what is the cause of the leaks...they just should not be there. That is more important than installing a pipe that can survive under incorrect conditions...and that pipe will be VERY expensive...
Working on steam and hot-water systems isn't rocket science....it's actually much harder.
Do you have a source online for this pipe, I get concerned that I see tiny leaks in my existing galv stuff around joints, and wanted to replace the pipe, and saw the very nice pics on here of the german home with what appears to be 8" galv pipework, which seemed to me that if I am going to replace this pipe it might be worth getting something very nice :)