They are old school flashlight bulbs, any hardware store has them. If I remember correctly, Regency utilized (old school) twin lead bulbs since they are soldered in circuit.
I may have an old one kicking around, PM me and we'll go from there.
From the service manual for the Regency ACT-R10 H/L/U, the bulb is a #73 rated at 14V 0.08A. This bulb has a wedge base and plugs into a socket. Mouser Electronics part number 560-73 looks to be the right replacement.