Rigid pipe is described by inner diameter (so 3/4" pipe is about 7/8" outside). Tubing ("soft" copper) is described by outer diameter. Tubing is usually connected by compression fittings, but you can solder it with the proper fittings.
See also How can I connect to copper pipe which is slightly oversized?.
Answer from isherwood on Stack ExchangeVideos
Rigid pipe is described by inner diameter (so 3/4" pipe is about 7/8" outside). Tubing ("soft" copper) is described by outer diameter. Tubing is usually connected by compression fittings, but you can solder it with the proper fittings.
See also How can I connect to copper pipe which is slightly oversized?.
ah d'oh .. it's 7/8" pipes. And yes they're sold online. The 7-11'ish (small-box, but close-by/convenient) Ace Hardware doesn't carry 7/8".