not an actual paid tutor, god no. Then you might as well just go to school
I disagree. Say you pay a grad student $45 to meet with you for one hour every week. They can make sure you aren't making any mistakes, point you in a new direction, and generally lead the way for your studies. This would cost a total of $450 over the course of a 10-week quarter-- Less than half the tuition for a math class. In return you get personalized attention. If you don't have any friends who can tutor high-level math, I would say paying for a tutor might at least be worth trying.
Also, as someone who self-retaught UP to calc level, I recommend the "X for Dummies", "X Demystified" and "Idiots Guide to X" books. They go higher than people think (but not grad school level), and include books on: Diff EQ, Linear Algebra, and Discrete Math. (I know you said that you personally have already studied these, but for recommendations for others, I think they are still useful.)
If you don't want to pay for tuition, you probably would prefer not to pay for a super-expensive text either. Don't forget to check with your local library, and if your state has an interlibrary loan. It's not uncommon that at least ONE library in your state will have the text you need.
Libraries routinely turn down ILL requests for textbooks or just academic works, as I've discovered multiple times. (Even academic libraries will do this if they think you're requesting too many papers, as I discovered while using a university library for Wikipedia & anime research.)
This will not be a long post; I have a simple question to ask: if you wanted to educate yourself to graduate level in mathematics, but didn't actually want to go to university, what would you do? I would ask for text-book recommendations, but I don't want to limit your responses (however, bear in mind that the wikipedia articles on, say, cardinality or well-ordering go over my head – they may skim my hairline, but over they go). Also bear in mind that while I personally have A-levels (British qualifications) in both Maths and Further Maths (which is to say, I know some calculus at least), there are probably plenty of people on lesswrong who don't and who desire the same information – so assume as much ignorance as you feel necessary (it's a shame, actually, that there isn't a sequence here on lesswrong for maths). What do you advise (if you think the query ill-defined, I would like to know that as well)?