If you're looking for resources for information about bondage and self-bondage in general that doesn't contain adult imagery and/or isn't restricted to your country's age of adulthood, I can understand the struggle. (No pun intended....Initially)
You've a

eady found one. I do like how LikeRa tries to keep most blog posts rather clean while remaining informative. This whole forum is actually fairly clean for the most part, so long as you stay away from the dedicated art and photos threads (run a much higher risk of adult imagery).
Another site I found that talks about rope bondage that keeps it clean is
http://www.ropeconnections.com. There are images of a woman in her skivvies, but that's still legal for the most part. These resources could be used to come up with self-bondage scenarios, as well as offer advice on how to work with rope overall. While this is not my personal rope tutorial site of choice (my preferred one has a NSFW/adult content warning at the gate), I have used tutorials similar to these to practice on myself and learn, and then eventually help to teach a willing partner how to do the ties that I can't do to myself.
While it doesn't really offer much in terms of advice,
the bondage webbing store has all kinds of neat products that are modeled on a woman wearing a zentai suit, which is still legally clean. I can't deny a certain thrill I get at just window shopping here. You'd also mentioned having a fair amount of disposable income, so this site would offer some things you could actually purchase and use in self-bondage. I myself can vouch for the quality and tell you that all of it is well worth the money. I'm sure others here that have bought some of Alex's creations could chime in as well.
What you're looking for is a difficult thing to find, but it's out there. You just gotta know how to work Google the right way, even with safe search on.
I will also say that I commend your dedication to keeping to clean materials while underage. I'm fairly certain I had enough freedom on a computer at the age of 17 that I could go find and download whatever I wanted without getting in trouble. You're a stronger person than I am in this regard.
The good news is you can't stay underage forever. Eventually everything will open up to you, and entering the adult world with experience in your own personal kinks and fetishes will give you an edge in staying out of some scary situations and/or finding a good partner to play with.