I cannot emphasize the importance schools place on having a strong math background for Economics Ph.D. admission. Out of the 11 schools I applied to, 4 had separate sections/submissions devoted to understanding the applicant's math background. You can check out the documents I had to submit here and here.

Take the calculus sequence (Calc I, II, III), linear algebra, and probability.  A real analysis + differential equations course would be extremely helpful, but the other courses are mandatory. If you were not able to complete these courses during your undergrad, I would suggest enrolling at Ahsanullah University's Math Masters program as a part-time student. You can do the courses you find relevant, and use their gradesheet to apply. It is a very cheap program, with classes on the weekend. 

While Policy programs will not have strict requirements similar to Economics programs, you can be sure other applicants have done a fair share of math courses.