Use them with JV team that I coach, Have a Comet, which is the team name. We give them out post game meeting, myself, Captains and girls can recommend, but not get too carried away, usually, not more than four or five per game and this has worked well.
They are easy to get you, any sporting goods store can order them for you, you can probably go on line as well. I do not like the ones for 2B, 3B, HR etc. Has been positive, players like them and it always makes a chance to get players to recognize each other and what was important in the game. We do, occasionally give them out for team effort (everyone gets one) of solid games overall etc. Interesting when you let players have say on what should be recognized how they seem to get a good sense of the game.
Costs is not bad, I was to say you can get enough for a whole season for around $50.
I umpire, have for years and it is beyond my compression that unless a sticker is a reason to disqualify a helmet, that an umpire would do so.