I'll be making these at the TechShop (link below) which has a ton of machinery and resources. One thing to note, I won't be going over the operation of each tool since you should've previously taken the safety/use classes. Obviously, this project is pretty simple in which you could do it with had shears and a drill OR a plasma cutter.... Anyways, here's my process.
Step 1: Buy some material
As for where to purchase material. Most all general stores have a small stock of aluminum but I prefer Alan steel in Redwood City, CA. Great prices and low cut fees.