How can I enter into the mathematics field at the graduate level when I only took a few math courses as an undergrad?
It sounds like your best bet would be to contact the graduate officers at the economics and finance programs you're interested in attending, and ask them what their minimum entrance requirements are. It's a much more efficient route than just deciding to take more math courses, and spending significant time, energy, and money on a quixotic goal.
There are several schools that offer a post-baccalaureate or "postbac" program in mathematics. These are typically one-year programs intended for students in precisely your situation: interested in pursuing graduate study, but lacking sufficient preparation from their undergraduate degrees.
Here are the top few I saw when googling "math postbac":
Smith College (aimed at women)
George Washington University (transitioning to a similar "Graduate Certificate Program")
Indiana University Kokomo