If your current purchased frames are still in good condition, you can keep those frames with your lenses replaced if your prescription changes. You also can purchase your own frames and the have lenses cut to fit.
Do you mean you just want the eyeglasses frame? If so, you can just buy eyeglasses frames only. You can get it both online and real shop. If you want to order it online, you need now your frame size. If you needn't prescription, you can also order plano eyeglasses that won affect your vision.
Yes, you can do that. But some optical shops will not cut the lenses for you. So you'd better take your frame to the optical shops. Actually I heard that some people will use their old frame to store the new lenses. But it often doesn't work as well as we think because old frames are easy to be damaged though the staffs in the optical shops are quite professional. They could hardly avoid deforming the old frames when they are taking off the lenses and then put on the new lenses.
Of course you can do that. Last month, my friend placed an order online to buy a pair of frame. And then he took his frame to an optical shop in the nearby. The optometrist cut the lenses to fit for the frame. But I think the glasses sold online are very cheap now. Why not just buy a pair of glasses online. Maybe buy a pair of glasses can save your money when compared with that buy frame and lenses in different places. Hope this helped!
I have done that before. I just love that pair of frame. So I decided to buy it online and then had the lenses cut to fit for the frame. But before doing this, I suggest you to have a good understanding of the sizes of your old frames and lenses. If the lenses are very thick, I think you'd better choose the proper size. Usually your current pair of glasses will tell you what size your frame has. If you can't find that information, you can call the internet seller and they will assist you to choose a pair of frame that will meet your demand.