You'll be lucky ....
The club, it seems, aren't interested in producing a half decent retro shirt (in cotton, if possible) for the fans to buy and the nearest thing you could get to that is the '68 Replica that you say you already own.
Occasionally, there are genuine shirts from the mid-80s (with the traditional three amber bands) that appear on Ebay that people have owned and are simply getting rid of them. They've usually got the Youngers sponsorship logo somewhere on them. Given that they are highly sought after, their price is at a premium and you can pay between £40 - £60 for a decent one. It's also worth noting that folks were generally a bit smaller then, so if you're looking for something bigger than traditional XL, you'll struggle.
There was a limited edition run of White Away Shirts done independently in the last couple of years (See you season ticket renewal booklet) ... I've no idea who did them

But do know they've not got any left and it is unlikely that there will be another run done in the near future.
There was another company advertising on here, but not seen their adverts for a while and it looked like their shirts were based on the traditional design, but made out of the dreaded polyester. Anyone know otherwise ?
Good luck in your quest .... You might need it !