I have 6 Motorola RMU2050's and they work pretty well for that sort of application. The headset connector seats firmly, and there's a 6 bay charger available. They're public frequencies, so no license required, and they have decent range for only being 2 watts. I don't use mine much anymore, they've just been sitting around for a while, but they're nice little radios. If you can bump up to a 4 or 5 watt radio (which require licensing) you'll notice a huge improvement in range, but the RMU2050 is about as good as you can get without a license.
They're around $200 each once you include accessories, so they're not nearly as cheap as what you're using now, but their build more like "real" radios.
Do you currently have a wired intercom system (Clearcom or otherwise) in your theater? It sounds like you're relying solely on radios for communication backstage. If you don't have a standard wired intercom already, I think that would be a better use of funds than spending more on radios. Radios are great for lots of things, but intercom is much better suited to theatre applications.
