Very confused by this. As far as I know it's standard for lenses to be custom made for your frame even when you purchase them at the same time.
I just sent an old pair of glasses to eyeglasses.com for new lenses. I never considered this to be a big deal.
> The frame will probably change slightly over time to make them incompatible.
This is probably true.
The rest of your comment is probably not true for most people.
It just depends on how strong your prescription is and how willing the shop/website is to do special orders.
If you are almost blind, then your choice of lenses/frames will be much lower than if you are only slightly blind(most people). Any reputable eyeglass/optician shop should be able to make custom lenses for pretty much any frame. They can't always do the super sleek shades that some people like to wear.
The frame will probably change slightly over time to make them incompatible.