On the links page of the
Softball Umpires web page is a selection of umpire equipment on-line dealers. The only endorsement the list implies is that at least one of our online umpiring community had at least one good experience in buying equipment at each of those. Beyond that, caveat emptor.
I can, by personal experience, though, vouch for Honigs, ASA Properties, Lester's, and Ump Attire.