Make sure whether your glasses have prescription or not. If they do have, you should go to any optical store to have an eye exam first. And then let the worker select the right lenses that will fit your old glasses frame well.
You can take your glasses to the local optical store and find the lenses are fittable for your old frames, then ask the optician to process it for you, but I think some of them are unwilling to do so for they will take a risk of breaking your old lenses when getting them out and putting new lenses in. Actually, it is also hard to find lenses that are available for your old frames, the best way to slove your problem is to buy another pair of glasses.