You can obtain a "low buck notcher" from American Fence co. It cost around 300.00 and it comes with the size of die you need and a couple of others. You can also buy other size dies from them.
They also sell a burn saddle that will clamp around the top of the pipe and use it as a guide for the torch tip to cut notch. Both work very well. If your pipe is greater than sch.10 you will have to use the burn saddle. I have used both products in my business for over 7 years and neither have failed me and neither have ever needed replacement.