[NM FP Ump]

I would try honigs.com or ump-attire.com. They have good selections of umpire safety equipment.

Get your pants from the ASA store; I hear they won't let you do any championship games unless you have ASA pants.

One ball bag, just make sure it's right color. Your best bet is to check out the ASA website and research the required uniform. If you are looking for ASA shirts, then the ASA website is the place to shop.