Where do you find problems to solve?(self)

Where can I find a bunch of problems I can solve for practice?

I would get a pdf of a textbook for the kind of material you're looking to practice

It depends on your level. You can always google something like "*Current Topic* Practice Problems and Solutions" to find questions related to the topic. From there, you can pick and choose which ones are around your level. If you're in college, sometimes the professor will recommend a book. If they did, you could look in that book and see if it has any practice problems.