Well, I know that *now*
The flat piece that feeds the pop bumpers or the flipper itself? The easiest way to replace a missing one is to get a similar one and adapt it. (unless you can find the exact piece of course) If the flipper has a wire guide behind it they are pretty easy to make with a vise or even pliers you can buy the raw wire at PBR or marco (or use welding rod of the appropriate thickness).
The flat guides are pretty easy to bend as well, you don't necessarily need the slip roller to do it, it depends on the thickness really. Got a pic of what you're trying to replace? I will look in my parts stash and see what I have.