But please link me to a US site where I can legally buy contacts without providing proof of prescription, if there is one!
That said, for normal eyeglasses, there are many sites that offer eyeglasses without proof of prescription. I've been buying eyeglasses from Zenni Optical for almost a decade for my whole family. Most recently in 2021. I have never once even been asked for proof of prescription, I just punch in my numbers and hit order.
One province de-regulated dispensing of corrective lenses and anyone could get in the game, and many did!
But good to hear you can in Canada!
The optometrist I went to recently put me in the strongest prescription I'd ever had. Instead of using that prescription, I ordered contacts using the prescription of the glasses that I thought were fine.
There's two numbers on the contacts... diameter of the pupil (doesn't matter for most people I think) and an angle (Edit: 'Base Curve'. The third number is the power of the contact, aka 'sphere'. Glasses prescriptions have power and astigmatism, but don't have diameter or base curve). I got these off my prescription. If you don't already know how to use contacts it's important to get instructions and a followup. I read of a guy who didn't know he had to take the contacts out at night - the contacts had started to grow into his eyes?
(Edit: you can get a set of "trial lenses" from Ebay if you want to try to come up with your own prescription, or to try out your prescription before buying glasses.)
My previous comments on ordering contacts:
https://news.ycombinator.com/item?id=13691768 / https://news.ycombinator.com/item?id=29366816
Edit: I live in the United States.