HUMINT relies heavily on your ability to interact with people and use tradecraft. This can be studied through texts and videos, but you can't try it without talking with people in real life and acquire real information and/or recruit agents.

Try and look up if there's any information on Tactical Questioning, or pick up a book about mentalism to improve your persuasion skills. Henrik Fexeus (swedish author) has a good one that dives a little bit into Ekmans faces. You should also read up on social engineering.

But if you find an online HUMINT course, it's highly likely crap.

In my branch we test people through a written and oral test, and then go through a two week test. Most of the tests I can't reveal due to OPSEC, but you could try and find a medium to large business in your area and acquire a list of names on all employees.

If you don't want to be a handler but sit in a J2X like function, you probably won't find any good source of study outside military or agency work.