3500 psi tanks are not all that common any more and it's not surprising shops don't stock them, but a 3442 psi burst disc will work fine so they don't really need to.
A 3442 psi tank has a test pressure of 5163 psi and the burst disk must fail between 90% and 100% of the test pressure, so within a range of 4646 psi to 5163 psi.
In comparison the 3500 psi tank has a test pressure of 5250 psi and the burst disk must also fail at 90% to 100% of the test pressure, within a range of 4725 to 5250 psi.
In practice the burst discs fail very close to the top end of the range when new, so a 3442 psi burst disc will work fine as it will fail around 5000 psi, well within the range required by the 3500 psi tank.
