What features should I look for in prescription eyewear for playing volleyball? What type of store could I find it at?
The most important feature are: a wide lateral vision (so bigger curved lens than normal glasses) and an excellent grip on the head.
I can suggest deformable plastic linked to an elastic to wear perfectly (only if you feel comfortable with it).
These featuresare not so prevalent because the athletes usually wear contact lenses.
Those two example with no elastic and elastic solutions to understand better.
There are two main things that you'll want to be looking at, the frames and the lenses. Ale has already provided a good answer as to what to look for in frames, so I'll just add a bit on what to look for in lenses.
The lenses should be made of a shatterproof material, such as a polycarbonate. You'll want to make sure the lenses are custom made to your prescription (duh!) and centered for your eyes. At least in America, most optometrists that also sell glasses will do this lens fitting for you and custom cut the lenses to fit your frame and eyesight. If you're playing mostly outside or with bright direct lighting, you may want to consider getting an anti-glare coating.
As to where to purchase them, most eyeglass specialists will have or be able to order suitable frames and install the lenses for you.