Pilot TV-37 CRT sub

Bought this little set at an auction last year--wonderful condition--and complete with the manual and nice carrying case. I've got raster, but no pix, so while I'm working on it, I'd just as soon use a scope tube just in case I "have an accident". I've got a couple, including an extra for an early electronic calculator but the number's gone and the other's a 5"-er. What're a few numbers in the correct size I can use?
Thanks!
Michael