If you want specific dimensions you don't want to buy pipe, you want tubing. You should be able to select a specific OD and then chose the wall thickness that will give you the ID. However, I have found fitting one tube into another is sometime too tight or won't fit at all. I have filed and used emory cloth to make things fit but on steel, that gets old in a hurry.
McMaster Carr is a good place but for materials the Metals Supermarket might be a better place to look.
McMaster Carr is a good place but for materials the Metals Supermarket might be a better place to look.