I recently established that "old" grade 1S aluminium is equivalent to modern-day 1050A, whichis 99,5% pure Alu. Does anybody know where I can get 3/8" tube in this stuff? Sheet seems fairly easy to obtain but not tube.

 

Try these guys... it may be custom... http://www.tubeco.com/index.asp

or look into aluminum tube for refrigeration...lots of china suppliers....