Each year as the golf ball manufacturers release new versions of their ball models, they discount the renaming inventory from the prior year. Since that inventory is limited, they are usually gone by early in the golf season.
Golf balls are expensive enough, but if you plan ahead you can save a good amount of money with buying a few dozen of the prior year models. Local golf shops may also offer these types of discounts but they usually have a very limited supply and sell out very early in the season.
Below are balls being sold by the manufacturers through Amazon, most are Prime eligible or come with free shipping.
Most discounts are around $10 per dozen. Here are the options with remaining inventory as of 5/7/19.
Srixon Q-Star Tour: $19.99
Srixon Z-Star: $29.99
TaylorMade TP5: $34.99
Callaway Chrome Soft: $39.95
Titleist Pro V1: $39.95
Titleist Pro V1x: $39.95