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I'm assuming that the "sac" that you refer to is the Winshield Washer Bag that was used on 69-72 Corvettes w/ A/C? Non-A/C Vettes used a plastic "bottle" type reservoir.
Since looking at your mods, it seems pretty obivous that you aren't looking for total originality. So you can do what I did and buy one from PepBoys, Autozone or just about any chain type autoparts. I bought one for about $9.00 that came with a little pump built in, I didn't use the pump, I just drilled a hole in the cap, ran a pickup hose through it into the bag. Then I hooked that hose up to the pump pickup on my wiper/washer pump. I also had to plug off the discharge of the little pump in the bottom of the bag to keep the water from running out.
The little bag I bought even had two brass grommets that matched up pretty good with the bag holder up under the driver's side fender. It was a cheap fix that works as good as the original and isn't noticeable to anyone but an NCRS judge.