If it's a round lens, I would imagine they could make it from dimensions, but a template/spare lens is more certain, to get the right amount of chamfer. I sent my old incorrect lens in its holder (bought my glasses S/H). Give them a ring and ask for James; he's very obliging and was grateful when I mentioned I had put a link on here.
When you visit the optician, he will give a prescription for near and far. Unless you know already what strength shooting lens you need, you'll need to get the optician to check the parameter for focus at the distance of your foresight with pistol raised.
Walther CP-2 Match, FAS 604 & Tau 7 target pistols, Smith & Wesson 6" & 4" co2 pistol, Crosman 1377,
Baikal IZH 53 pistol, Gamo CFX Royal,177, Umarex SA-10 CO2 pistol.