I'm not sure about HN policy here regarding linking to such things. For a couple of years I ordered from Afinil Express, who have semirecently changed their name to Shark Mood. You can guess/Google the URL. For security you'll definitely want to use cryptocurrency and not a credit card; apparently the issue is shady overseas card processors and not the merchant(s) themselves FWIW.
As far as dosage, it's really trial and error I suppose. Generally each pill is meant as a single dose AFAIK. You can always dip your toes in the water by starting off with 1/4 or 1/2 a pill.
I generally do 1/2 a pill before breakfast and 1/2 a pill before lunch (on work days) and roughly half that amount on off days.
Gf had a huge stash she ordered from India years back. I almost ran through them over the years so will need to restock but it doesn't seem as big of a deal as getting rec drugs, more like ordering steroids
The training that good judgement project gives you is a beautiful introduction to all sorts of evidence weighing, prediction, black swans, power dynamics of an outcome etc. Should be required for more professions and possibly democracies
EDIT... and it looks like this is a controlled substance. I didnt know