Hopefully I posted this in the correct section.
I have NO idea where to locate a rear sight for a Intra-Tec KG-9
First, you need to be certain of what you have. The KG-9 was manufactured by Interdynamic not Intratec. Do you indeed have the KG-9 model which fires from an open bolt? The KG-99 (also manufactured by Interdynamic) looks identical but fires from a closed bolt. Interdynamic changed their name in the mid eighties to Intratec. Intratec's firearms were the Tec-9 an DC-9 and I do believe that the rear sights are somewhat different on all of these various models from the Miami company. Can you supply a photo of where the sight was mounted?
Depending on what model you do have, you may want to try this vendor for parts.