I don't know if you found one or not, but forget about the FBA-15. Do a search for FBA-21. It's the same thing as a 15 but holds 6 AA batteries for a total of 9 volts, unlike the 15 that only takes 4 batteries.
You can find these packs on aviation websites being they are used for Yaesu's line of NAVCOM radios that are equiv. to the FT-50R.