I got one of the local high street glasses places (SpeckSavers) to make me bi-focals. 1pair clear, 1 pair brown tint (brilliant for overcast days) and 1 pair dark grey (for sunny days...well we might get some here's hoping). Did 1 pair first to check that the bifocal line was just where the cowling and windscreen met. They had to remark and slightly raise; but did it at no extra cost.
Remember to ask for the sunglasses to be NON-polarised.....you want to catch that glint off that other aircraft in the distance well before it and you get too near together!
Do not get varifocals as in the middle there will be a neither one nor the other prescription.