It is generally not super difficult - most professors at universities have Masters and PhD level grad students, and more often than not, faculty are listed on the university websites.
Find a local university, look up their math (or whatever) department, and a simple email to one or more professors explaining what you want to do and that you're willing to pay will more than likely give you some leads if nothing else. Hell, you might even find that the professor's time is within your budget depending on the institution and their level of seniority.