Not available? I was at Larry Stock's shop in Carson City, NV last week. He showed me at least 50 examples his machinist had finished up. These are the fully tubular type a-arm that use a 5/8" SAE fine thread heim-joint and jam nut to make rear camber adjustments. But I suggest you first examine your car's rear suspension to find out what changed that requires more adjustment than the stock shims at the lower rear a-arm mounts can compensate.
If its the usual slow collapse of the Pantera's rear chassis over time, know that is correctable if you use the type of adjustable upper rear brace-bar that has long ends to jam solidly some distance into the fender bracket weldments. This type is the black steel bar once marketed by Hall Pantera (and others today; Larry makes those, too). It's design forces all cornering loads to be absorbed by the welded upper rear shock mount, with the stock (flimsey) bracket and slotted mount holes being relegated to simple bar retainers rather than cornering load points.
Adding a gradual amount of preload to that type of brace-bar allows an owner to 'tease' the rear suspension back to stock alignment parameters. Then adjustable upper rear a-arms can be used to trim the rear wheel camber and the stock lower shims used for setting rear toe-in, all for better handling. One should NOT add all the preload at once or ripples can show up in the rear quarter panels. Don't be impatient! Aluminum adjustable upper brace-bars with short ends overstress the thin mounting tabs if used to carry loads, causing the adjustments to slide back & forth in the stock slotted holes so are far less effective.
NOT restoring stock chassis alignment first means the rear suspension will need more and more adjustment over time, especially when oversized rear tires made of stickier than OEM rubber are used, and/or when the car is driven in the manner envisioned by designer Dallara. Adding more and more rear camber adjustment pushes the rear tires outboard each time they're straightened up, until the rear fender lips contact the big tires and forces those lips to be pounded under (or flared) the same as max-oversized front tires that contact the front fender lips (while turning). There are illustrated details of this mod' in the POCA Archives. The parts are not expensive.