In the past I have used Khan Academy to great success to practice advanced math problems. It is useful because it gives simple questions in a multiple choice fashion then it quickly tells whether your were right or wrong, and it will walk you through the correct approach as needed.
Now I am in my first semester of a PhD in economics. Khan Academy has a series on economics, but it is no longer sufficient. The series is not accompanied with practice problems and it is done largely in algebra rather than calculus. Finally, there are a number of graduate topics it entirely misses such as:
- Phase diagrams
- Lagrangian multipliers to solve for optimal allocation
- Compare/contrast of using different models for the same question (Austrian, Solow, Diamond, RCK, DSGE...)
If it helps, a primary course text is Romer's Advanced Macroeconomics, 4ed.
What are some online resources to practice such problems?