Find a local mom & pop optician shop - NOT a chain store: the mom & pop will be happy to dye lenses in a pair of glasses you already have, or swap in new lenses in a frame you have and like. Or sell you a new pair. The "name" brand sunglasses are truly nothing special, except you get to pay for the name.
If you have single vision lenses just about any place can fit them well enough. If you are using multi-focals, or have an astigmatic correction, or other complex prescription, run away from the chains, especially in the states where the optometrists have kept the opticians from being licensed.
Heads and faces aren't symmetrical, and in complex prescriptions a trained and/or experienced optician/optometrist can get the optical centers correct. They actually have to know how to measure PD and adjust the frames on delivery.