I buy all mine here, about a quarter (or more) the cost of store bought:
https://www.zennioptical.com/
I presume you have the prescription from your Dr.?
You will also need pupillary distance (PD)
Don't want to be captain obvious but you have to get all those number exact or you will literally be getting a headache.
You can get PD rulers but I just put on a a pair of glasses, got close up to a mirror, and made a small dot over my pupils with a sharpie (then cleaned glasses with alcohol) and measured that..
You will also want to take a metric ruler and measure your head to know what fits and what doesn't. Once you have those numbers you will be able to use them in the future.
I typically get glasses for around $50 from these guys.