There are one-to-one tutoring services that target university students, and I doubt they'd have any problem with a paying customer who wasn't a student. Some of

There are one-to-one tutoring services that target university students, and I doubt they'd have any problem with a paying customer who wasn't a student. Some of them offer tutors who have PhDs. From what I can find on Google, prices range from "affordable for a student" to "affordable for a student with rich parents if they only need a few hours".