If your prescription is difficult (i.e. you have astigmatism, or strong correction, or sensitive eyes) I suggest you get a fully custom mask.
My wife had one made here:
http://rxdivemask.com/ I ordered it over the phone and it arrived when promised. The mask was a Tusa Splendive II, which she tried on at the LDS to make sure it fit OK. You can send them any mask or buy one from their stock. The price was about $189 for that particular mask, purchased from them, with her prescription. The lenses perfectly match her prescription and are bonded to the inside of the mask using some kind of optical grade adhesive. The results are quite professional.
Personally, I use a HydroOptix Mega 4.5dd. This amazing mask corrects underwater vision without corrective lenses. I know it seems like snake oil, but it really works. If your prescription falls within the "naked eye" range of this mask, I highly recommend you try one. Their website is
http://www.hydrooptix.com/ ... I suggest you take some time to read about it and understand its advantages. It will give the best view underwater, period, of any dive mask on the market.
HydroOptix also make custom-prescription flat masks for about the same price, where the lenses are pefectly matched to your prescription rather than "bonded." This is what I would get my wife next time. (She is not a naked-eye candidate for the Mega 4.5dd mask.) The custom masks are sold directly. The Mega mask is sold through a dealer network, or directly if there's no dealer in your area.
If you have a weak prescription and/or no astigmatism, you will probably get by fine with the replacement lenses sold by Tusa and others.