Before you spend a lot of money on a pair of medical glasses, try your local hardware store and see if they sell the same coloured glasses as safety glasses. I got a pair of yellow safety glasses, with yellow supposedly being good for astigmatism and car headlights, but only paid about £3 for them. 'Medical' glasses, which were the same colour but in nicer frames, were a lot more expensive.
I've also recently bought a pair of red glasses for about £5. They're not the prettiest glasses I've ever seen, but they work to let me know whether red glasses work for me or not :)