Comment by johnwalkr

Misumi sells it, and they have an online tool to configure almost anything you want in 3D, including brackets of your choice, fasteners and accessories (like panels, hinges and handles). Then, you can order all of the parts to make your creation with 1 part number.

Makerbeam is also nice, they sell a 5x5mm, 10x10mm and 15x15mm version (normally the smallest cross section of t-slot is 20x20mm). The 5x5mm version is comically small when you hold it in your hand. When you order a set, it comes with tweezers to pick up the screws which are size M1.2.