How much correction are we talking about? If you don't need much, you might get away with nothing at all, since a certain amount of magnification comes from being underwater.
Depending on the mask, some manufacturers also sell drop-in replacement lenses with different diopter corrections, typically in 0.5 diopter increments. Under water you can get away with a less exact correction. Look for masks with two separate lenses as opposed to one big piece of glass. Doesn't help with severe astigmatism.
There are several outfits that have been mentioned here on Scubaboard which specialize in custom grinding lenses. Send them your mask and they'll send it back with a fully custom correction. Some people have also been lucky and found local opticians who've been willing to do the same. A couple of weeks might be a bit tight, though.
Some people use stick-on lenses to the insides of their mask.
Quite a few people wear contacts under water. Doesn't work for everyone, though. For some people, they tend to pop off if your eyes are immersed in water. For others, it's not an issue. Some people prefer using one-day disposables, to avoid the inconvenience if they do loose one. I think it's a common recommendation to clean and sterilize contacts or dispose of disposables right after the dives are done, especially in sea water and particularly in the tropics. Lots of nasty creatures can get trapped in there and breed happily in the warmth of your eye.
BTW, it might also help if you
filled in your profile to include your location, so that people in your area might be able to tell you where to find things.
