First you need to attempt to determine if Zerks are metric or inch threads. On my Kubota Grand L, assembled in Japan, the chassis Zerks are uniformly metric. The Kubota loader, manufactured in Georgia, has inch threads, so far as I can determine. You may or may not find yourself in my situation.
Tractor Supply Co. and Agri Supply both stock metric and inch threaded Zerks, in single size packs and mixed assortments. eBay is another source for Zerks, especially odd sizes and configurations. Zerks are cheap.
Buy yourself a grease gun coupler. Users think couplers are overpriced but are happy they spent the funds and would buy again. Easy on, easy OFF. Coupler decreased my FEL greasing time by at least one third, while it improved grease distribution.
LINK:
Amazon.com: LockNLube Grease Gun Coupler - Zerk Grease Coupler Fitting Tip - Easy Lock-on, Clip-off - Stays on - Grease goes in, not on the machine: Automotive