Snorkeling: Where can I get a swimming mask that will let me use my prescription glasses to see well underwater while snorkling?

For a snorkel or dive mask to be most effective, they must have an airtight fit. You may want to consult a local dive shop. I am sure many have dealt with this issue. You may also want to try this link: http://www.shopwiki.com/Custom-Prescription-Dive-and-Snorkel-Mask

Using your actual glasses underwater is difficult unless you get some kind of elaborate full face mask, which won't really work and then you'll look like an alien and probably lose your glasses anyway.  Show your prescription and your mask to a dive shop.  The shop can order prescription glass lenses for your mask and replace the lenses you have.  It really depends on the mask, and if you need to, buy a mask from them that they can fit these lenses into.  Depending on the dive shop, your prescription and your mask, they may have the lenses on hand and swap them out in about 15 minutes.  If they have to order them, you're looking at a week but again the swap-out will take only a few minutes   There are also small plastic inserts which you can place in the lower part of each lense just for reading camera screens, settings, etc, but more importantly if you are scuba diving, the various gauges that you need to consult.   These you can glue in yourself and it takes about 8 hours to set.  I've used them, but at a certain point, if you're using your mask a lot, they fall off, especially if you are using any form of mask defogging liquids prior to dives.  Basically that stuff dissolves the glue.  These things are cheap.  There's no reason to go looking for an invisible plastic lens underwater.  Hope that helps.

You can also consider single-use (daily) contact lenses.  Although they may not be good for every day use, they might be good enough for swimming.   I used to do this: I used contacts with my swimming goggles; and glasses for the rest of my life.  I couldn't drive with the contacts; I just couldn't get the proper corrective diopters I needed along with some other issues I have like astigmatism. Daily contacts are probably not as expensive as a custom-made mask.

I have just bought a pair of IST Corona with -2 glasses and I do not think they are so pricy. However it was a much greater experience when I actually was able to see something at our last vacation ;)