First of all - how complicated prescription do you need?
If you only need plain minus glasses <8 (or 10), go to you dive shop and try the models they sell with drop in lenses. Most shops will have a few. Some nameless cheap ones that might be fine, most likely something from Tusa or Cressi (Cressi Big Eyes is one that fits a lot of faces and is a very popular mask) etc.
If you need plain reading glasses, you might be able to buy inserts for your mask that are just glued (floated) on from your LDS shop as well.
These options are generally the most affordable and work fine for most people - especially as starters while you find out what mask fits you best. (Remember you first need to find what mask fits you best whatever option you choose anyway. Do not order expensive Rx lenses for ANY mask. ANY mask will NOT work. You need to try it on).
However, if you need complicated prescription (bifocal, astigmatism or any other crookedy eye combo) you are better of looking at the links people provided in this thread.
And remember if you just have minus lenses with strength less than -2, you might not need glasses at all under water - and you can also consider using contact lenses. Good luck!