I've been happy with these. Inexpensive, comfortable, and available in several colors/shades. I think I found mine on the shelf at Academy Sports but, clearly, they could be ordered from any number of places.
It will vary depending on the style & shape of your prescription glasses. I like big "aviator" types & have never found one to fit.
I usually just go to those Health & Safety mega stores online & buy prescription safety glasses ground to my scrip. They're surprisingly cheap as they're required by fed regs as issue for places where safety glasses are mandatory.
try here: http://www.safetycompany.com/categories/safety-eyewear/rx-safety-glasses/
If the lenses on your prescription glasses are big enough to cover your whole eye, you need nothing else. Prescription glasses are impact resistant.
The big "aviator" types that all us big, hairy, men wear are fine as is. Side protection can be added with add-on side shields. http://www.safetyglassesusa.com/sideshields.html