Hard time finding steel rear brake line that goes into driver side rear rubber brake hose

It should be possible to make one using a rubber line and a section of steel or nicopp line to form the metal section. I rarely buy the preformed lines as they can be a lot more expensive than making them and many times you need to snake them into spots that a preformed line can get deformed in. The only tricky part is making the proper flares. For that a great tool is a flare tool like the Cal-Van 165 Inline Kit. It does a good job and doesn't break the bank.
With the GM strike over the parts might become available now but maybe not. The part number is ACDelco #10408539

Okay, you need the rear section that goes to the flex line and then out to the wheel cylinder?
I don't show them as available from GM at this time. The rear flex lines are available but not the steel lines that go from the front line you have already.

GM just shows the part number for the end fittings and a coil of 3/16" brake line. No part number for the complete line. They do that a lot when they discontinue a part.

This should help:

https://parts-catalog.acdelco.com/catalog/catalog_search.php

let us know if you need anything else.

Here is the entry from the GM factory listings for items 13, 11 and 7 which are the rear brake lines. They all use part number ACDelco #12547207

PIPE, ELEK TRACT CONT BRK PRESS MOD VLV(3/16" X 20 FT ROLL)(CUT TO LENGTH)(08.964).
PIPE, BRK(3/16" X 20 FT ROLL)(CUT TO LENGTH)(08.964).
The fittings (number 5) are 3/16 male line nuts.

ACDelco #12547207 is a 20 foot coil of 3/16 brake line that you use to make the replacement line out of. It is the left picture It is not a pre-made part, you have to make the line yourself by adding the proper line nuts and the flares. There is no pre-made part available for your car. That is because auto makers only need to provide replacement parts for 7 years from the build date of a vehicle, and unless it's something like a classic or collectible car, none of the aftermarket companies will make the parts either because they don't want to make 500 parts and then sell 5 of them. If you look in the service books they all say the same thing about brake lines, get a coil of line in the correct size and make the line. The only time that changes is if you have a vehicle that is still using the same part or they still have those parts laying on a shelf somewhere. In the case of your car, the rear lines tend to rot off in many places so the dealers have used up the stock of OEM parts and now make the lines as needed. There are a lot of videos online about making them, everything from a DIYer using the clamp and bar flare tools to shops using benders and high end turret flare tools to make them. It's not hard to do but you have to follow the steps correctly to get a good flare. And whatever you do, before you make the flare, be sure to put the line nuts on and in the correct direction. BTDT at least once.

Another option would be to go under the car, measure the old line from fitting to fitting following in so you get the correct length. Now go to a parts store and get one of the pre-made straight universal lines (right picture) they sell that is the correct diameter and as close as possible in length, say yours measures 23 inches and they have 24 that would work, just don't go shorter, then bend it so it fits into place. If it is a lot longer like your line is 23 inches but they only have 36 inch line, you could use it and bend it into shape and only need to flare one end. Some of the old time parts store might even make you a new line if you took the old one off and brought it to them. I make lines for folks quite a bit because they rust off fast where I am.