I suspect that you will have difficulty in finding standards - I ran a mechanical testing lab for stainless sheet steel in a steel mill for 5+ years, and even with the longer history of steel had to develop standards internally.
In the hope that my program might help you devlop a similar one for your product I'm giving a detailed description of it.
We used commercially produced testing equipment to measure tensile properties of the sheet produced by the mill. We were unable to find a traceable standard material to use for verification purposes. What we did was have the testing machines calibrated by an ISO Guide 25 calibration lab on a regular basis (I left just prior to the upgrade to ISO Guide 17025 from ISO Guide 25), internally developed a standard used to verify the machines each turn prior to use - basically took a scrap coil, verified its test results throughout the length of the coil & across the width of the coil, had that coil cut into sheets about 4 ft. long, placed those sheets on a pallet and stored them in the lab so we could prepare standards when needed. We ran SPC charts to verify the results of tensile testing equipment each turn prior to use - took 2 standards, ran them, plotted the results on the chart using averages & range between the 2 individual specimens for 0.2 % offset yield strength, ultimate tensile strength, and % elongation. If results were within the spc limits, the machine was OK to use. If not, we had a work instruction describing what operators could do prior to getting the supervisor involved. The machines had to be verified as OK to use prior to running samples for verifying coil properties. As part of our program to verify the machines & determine capability, we also took part in interlaboratory tests run by an outside concern - received material to prepare and tensile test twice a year and had to report results back to the outside concern which published the overall test results twice a year.
One thought regarding accuracy would be to contact a calibration laboratory capable of verifying the equipment in place. For the tensile testing equipment, I used Tinius Olsen. I believe Instron also offers a similar service, and there are others as well. My thought for setting up your equipment would be to calibrate in place by an external service, secure a proposed standard material prior to their calibration, run the standard to develop base SPC data, and have the equipment recalibrated/checked by the service. Then use your internally developed standard to verify your equipment as required by your production practices.
By the way, do not expect to get the same results from each piece of test equipment - if you have adequate discrimination in your test, you'll end up with similar but different results.
Hope the above helps you towards a solution.