Component suppliers in the UK
eBay is always worth a check first.
Boiler tube;
Marine Engineering Pipeworks (Site not working at 5/6/2024)
Note, it is quite economical to send 6m pipe lengths by Tuffnells
but I have not so far found any carrier for 7m lengths.
Imperial sized Cunifer 90/10 is sold in 25 ft rolls as brake pipe, up to 3/8 inch diameter.
Fittings, 1/4 BSP and up;
Fittings, under 1/4 BSP;
Stripwood;
Cedar Strip