I just made some from strips of brass manufactured be K & S Engineering Which I purchased from the hardware store. I used basic had tools including a hacksaw, cordless drill, needle files, and a small combination square with scribe. I used a Sharpie marker for layout dye and then used the scribe to carefully layout the location of the slots. Then drill a series of holes and used the needle files to elongate the holes into a slot. In other words what mjs said.
I did look at Dremel tools while at the hardware store purchasing brass, but the Dremel just wasn't in my budget.
The clips btw are to hold the back on a pinhole camera that I'm building. (there was a url link here which no longer exists)
Roger