Conveniently placed near the spawn
Simply put, next to the hospital, at the back of the hotel.
Show me the way
The main window
Browse the different categories with the tabs at the top.
When you enter a tab, it might say that you don't have access to it.
For certain items, you might need a job that has access to buying them, you can see this type on the menu on the Business tab, and set it at the estate manager.
Protip: Buying more than one item at a time usually means cheaper bulk buying.