I know what you are talking about puppetgospel and they are really hard to find anymore the harder foam (like golf practice balls) but larger in size and usually for crafts....I have a couple boxes that I picked up a few years ago that I have around when I need them however they are really hard to find anymore.....I wish I had an answer for you.   If you find any please let me know.

Daryl H

Ballistic nerf ammo balls are a little bigger than golf balls though not much.  You can usually get them in Target or Walmart in the toy department.

Try looking at bobbers for fishing.  They're not the same material but they may do the job.