Up until four years ago, we thought we'd identified the holsters used in
Ghostbusters, but turns out we were wrong. Jairus managed to ID the holsters used on the costumes
here, which he speculated was a holster made either for the Motorola HT440 or HT90.
However, they come with a major downside, with over 30 years of age, the most screen accurate holsters out there become a non-viable option because they appear to be super brittle.
Your best bet would be to keep an eye on eBay for a genuine MT-500 holster, they've what the community has been using for years, and they seem to stand up to the test of time... The tricky part is that between the market being cornered by the Ghostbusters community, as well as other historic costumers, like Police reenactors, and because they're not being produced anymore, they're harder to come by.