I used EyeBuyDirect.
But their no-name brand frames suck ass and fall apart quickly, you're gonna need to pay more for the quality frames so they don't fall apart in like 3 months.
I don't think online glasses stores are gonna save you much money, its more like a convienience thing, so that you don't have to wait in a physical store. You're still sacrificing quality and durability with price. Those cheap frames will fall apart much sooner than the expensive ones.
But, I use them because they have some frames that is ready to ship within 2 days, great for when my glasses break and I needed one like... ASAP.
Stores take like 1 to 2 weeks, I hate that, takes forever.
I don't think there's a "best" way, its just what's convienient to you.
Zenni Optical and EyeBuyDirect are the most common ones I read about on the internet, and the top 2 search results, but since Zenni didn't have 2 day shipping option, EyeBuyDirect is what I chose to go with.
EyeBuyDirect has like a one time free replacement for defects. You take photos of the broken glasses, send it to them, and pay shipping, and they send you new ones, they didn't even want back the broken ones (I mean, its not must use to them anyways). (But don't choose the same frames if you are gonna get a replacement)