You cannot buy prescription lenses without a valid, current prescription.
Often the most cost-effective option, with complete pairs starting under $80, including basic lenses.
This is the distance between your pupils. It is essential for ensuring the center of your lens matches the center of your eyes. You can ask your doctor for it or measure it yourself, though a professional measurement is preferred.
Services like Rx-able.com can replace lenses, with single vision lenses starting around $28 and progressives around $58.
If you love your current frames, you don't need to buy new ones.