You've asked the million dollar question, and the hurdle for any of the degrees that you plan to complete on your own and transfer back to one of the big 3.
It's super easy to find cheap / free sources of credit for general education courses at the 100/200 level, and even for electives or business classes- after that you can either go for the easiest (most readily available upper levels) which would result in a degree in Liberal Arts, Business, Social Science, or even Psychology, History, or Literature.... but the rest of the majors are more tricky. As you are finding, it's no easy feat finding upper level courses to transfer back, and if you find them, they may be a little pricey.
Any 4 year university that has an "Extension" campus is a good place to start- mainly because you don't have to apply as a degree seeking student to register for an upper level. Many universities want to limit those courses to their enrolled degree seeking students, which can be tricky.
A few I know off the top of my head:
Harvard University
University of California Berkeley
University of Texas - Austin
Brigham Young University (Independent Study)
https://is.byu.edu/site/courses/index.cf...n|paper=on