I use 12 volt batteries, type A23. They are readily available much smaller than a AA and last for a long time when providing B+. The holders are available on eBay cheaply. 5 in series would give you 60 volts which will work just fine. You may find the original battery is still available. The B+ battery for my Motorola Pixie is still available, although they are very expensive.
https://www.lowes.com/pd/Exell-Batte...ies/1000682769