Majestic has discontinued their umpire shirts. You would have to acquire them on the used market like I have.
Smitty, Adams, and Honigs have come out with their own iterations. Adams is definitely the discount version right now. Smitty has had a v1 and a v2. The v1 was absolutely awful, in terms of the cut, the fabric, the quality, etc. The v2 is a closer copy of the Majestic and noticeably improved. I've heard good things about the Honigs.
This would be a great @MadMax question!