Guitar Jones has 53mm 50mm and 49.2mm bobbins. The 50 mm bobbins are slightly short (vertically) and very shiny, while the 49.2 mm bobbins are very tall, enough to put at least 5.6k worth of #42 wire on one, but the finish is duller.
Allparts makes nice 50 mm bobbins. They may make other spacings too but I didn't ask.
Both GJ and Allparts have price breaks past a certain order size.