Where can I buy pure materials?

I want to study Physics and Chemistry, but I'd like to be able to apply what I learn as well.

I want to make things, by understanding what things are.

The issue is finding pure forms of the elements. For example, I'd like to work with Silicon or Copper or Hydrogen, but I'm not sure where to get them.

I know I could find them around (like Silicon in circuit boards or Hydrogen in the air), but I'd rather just purchase already separated units.

Where can I get pure materials?