Okay, in my quest to try to have one bike I'm happy with, I figure I'll need to buy a new bottom bracket along the way. My common sense told me that you probably measure spindle length in millimeters, however, mine measured 125mm, and I'm not seeing that size anywhere. My bottom bracket is an old Sugino, unsealed.

Thanks guys.

i think that's exactly how you measure them https://sheldonbrown.com/harris/bottombrackets.html

Are the crankarms held on with nuts? Older BB's have threaded posts sticking out past the sqare ends of the tapers. When you measure the spindle length, just measure to the end of the taper and ignore the threaded bit.

with a caliper or ruler

Well, I measured mine, with a ruler, not including the threads. I didn't see on that page how to measure them, but maybe I just need to go to bed.

I wonder if I can get a sealed one?

You can find 125mm sealed bottom brackets. This width was commonly used with some older cranks; the spindle length on those tends to be longer than that on more modern cranks. I know you can get them on biketoolsetc.com, and likely nashbar as well.