Where can I find a collection of past exams w/ answers on various subjects?

MIT's Open Courseware project has a lot of material of this sort. Here is a link to the Course Finder, with courses organized by topic. The depth of the offering will vary from course to course. Some have only syllabi, others have complete video lectures, homework assignments, exams, solutions, etc. The coverage is quite broad and includes courses in all of the specific classes you have listed.

Search for courses, you'll find lots of material. Common are (more of less polished) lecture notes, often you'll find homework (sometimes with answers), also exams.

Coursera provides free courses in many different fields from University professors. They also offer you specialized programs and certificates when you complete them (certificates are not free, but not expensive either).