If I can add to the EXCELLENT advice from Rick, I would strongly support his suggestion you consider changing your whole fleet over the DLR1020 or DLR1060 radios.
http://www.buytwowayradios.com/products/motorola/motorola-dlr-1060.aspx
Unless you have a need to tie in to other GMRS or FRS radios, the Motorola DTR/DLR radios are high-quality business-class radios that will provide many hours of battery life and will survive drops to concrete and even some rain. They cannot be monitored by anyone else on consumer-level scanners, and the only radio who can monitor them is another DTR or DLR radio. (And with many thousands of possible channel IDs available, that is not likely to happen.)
The DLR is particularly compact and unobtrusive.
They would fit your needs exactly. It would mean changing your whole fleet, plus new accessories, but you can now access a WIDE range of professional-class headsets and accessories that are much higher quality and more durable than the more consumer-oriented FRS/GMRS radios.
So if you just want the cheapest solution, any FRS radio will work. (The frequencies are common among all FRS/GMRS radios, and the frequency numbers are consistent among most manufacturers.) But if your budget allows, a fleet of DLR radios will be compact, durable, industrial-quality and very private.
I have used the DTRs for years and buytwowayradios graciously sent me a sample of the new DLR radio. (My reviews are on this forum and on their web site.) I am now a HUGE fan of them. You don’t have the same ability to call up individual radios as you do with the DTR, but I almost never use that feature anyway. If I didn’t have a fleet of DTRs, I would definitely go with the DLRs.