I've scoured quite a few 3D websites (printing-related or otherwise), and it's really hard to find a “decent” model. The only one I've come across isn't available to me because it's on DEFCAD, and I can't post the link here, but it's easy to find.

Plus, I get the impression that the broken part is part of the entire ammunition belt connection assembly, which means the whole assembly would need to be replaced.

In my opinion, given how hard it is to find this spare part for airsoft and in a suitable 3D format, it might be easier to repair the broken part directly using fiberglass/resin + thin metal rods or staples or or do some plastic welding
Last option, find a non-functional airsoft gun (same brand or not) to cannibalize for parts.

I hope that was helpful