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I removed a leaking (rotted) rear brake line for my right rear wheel. It is about 40 inches long, and 3/16" steel brake line with a M10x1.0 male connector at each end, but the flared ends are different.

One end has a long narrow connector with 3/8-24 UNF (SAE) thread with inverted flare (also known as a double flare or SAE flare) that was screwed into a female-to-female adapter near the center of my car.
The other end has a short connector with M10x1.0 thread with bubble flare that was screwed into the end of the short brake hose for my right rear wheel.

Ideally, I'd like a 40" 3/16" steel brake line with an M10x1.0 Male ISO Bubble Flare connector at one end (to connect to the brake hose for my right rear wheel), and a 3/8-24 UNF (SAE) thread with inverted flare at the other end (to connect to an adapter near the center of my car). Maybe the 3/8-24 UNF connector with inverted flare was the wrong one originally, I dunno. I've read that for 97 GM cars, all the connectors were M10-1.0 bubble type.

Yeah, I know it sounds weird having both ISO (European) and SAE (USA) flared connectors in the braking system, but I think it may have been common for 90's GM models that had mixed metric and Imperial nuts&bolts too.

I would appreciate any recommendations on where to buy a finished steel brake line plus an adapter or two if needed.

AutoZone or the like was the easiest way for me when I was replacing rear lines. I did have to cut an end off their pre-made straight line to change the fittings at one end and have a manageable length. But it worked out. Or maybe try Amazon. You might pay more, but there could be more options with better anti corrosion coatings. I think I might have done that the 2nd time around.

So I found my parts spreadsheet from 2011 when I re-did the whole brake line system on my 95. I remember the majority of the system being mostly European bubble flare. That female to female adapter in the center of the car is a proportioning valve (p/n 19244448) and there is one per side.
I ordered 50' of copper nickel line and all fittings from here: https://store.fedhillusa.com/316475mmtubingandnuts.aspx
The #6 nut pack with assorted European metric fittings covered most of the system.
"Common metric steel nut assortment for European cars 3/16”4.75mm tube -14 pieces
DIN (bubble/ISO) flares
8 ea M1-3 standard metric European steel nut - 10mm x 1.0 non-threaded lead - 17mm long
4 ea M1L-3 standard metric long European steel nut - 10mm x 1.0 non-threaded lead - 23mm long
2 ea M6-3 standard European master cylinder steel nut - 12mm x 1.0 non-threaded lead - 20mm long"
I also ordered 4 of p/n M6-3 (12.0 x 1.0) presumably for the two proportioning valves.

I can't recommend copper nickel line enough. You can bend it by hand, it's exceptionally easy to flare and it'll outlast the car.