Any grease gun will have that fitting. I simply unscrewed it from one of my older grease guns, took it apart and cleaned out the grease.
Just looked online. Grainger has the fitting for $2. Many others between that and $5.
I used a piece of quarter-inch OD static/pitot tubing. It slipped inside the tubing called out in the DIY garden sprayer bleeder plans (safety wire to secure), and the ID was perfect for a slip fit over the bleeder zerk.
The advantage of the grease gun fitting is it is a check valve so you won?t spill any fluids.
However, as little as you need it, cleaning up a few drips of fluid now and again isn?t that big of a deal.
Use whatever works.