There are devices to measure IOP, most used but kind of inaccurate with a small „air burst“ on your eye. There seem to be more accurate measurement methods but they involve „touching“ the eye.

Can an amateur buy them online?

No definitely not. Your doctor has it. See https://en.wikipedia.org/wiki/Intraocular_pressure

well it can actually be bought for 700$ but yeah doctors are a thing.

also: > Differences in pressure between the two eyes are often clinically significant, and potentially associated with certain types of glaucoma, as well as iritis or retinal detachment.

> Berberine prevents myopia progression

Any sources about that?

I edited ly comment, it's not Berberine it's Bilberry https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6296196/