Spur Gears: Cheap Perhaps Scrap Source?

I am looking at the slow motion declination axis control.   This could be a worm gear or two spur gears.  The exact reduction ratio is not terribly important.   A slow motion control is used by an astronomer to make small changes, fractions of a degree at a time.   A worm gear is nice but worm gears with a .5" bore diameter are shockingly expensive.   A couple spur gears could do the job.  There are devices out there likely scrap that have slow gears galore in them.   Buy what sort of devices?