We use the Korg TM-50. Its the one that also has a metronome. We put them in the calculator pockets on the wall. I know there are other cheaper options, and I am sure your local store can give you other options that they can procure, but this is what we have. Honestly, we have been doing it for years, so the only difference is just that we have more of them now.
I make sure to go over how to use it, settings, etc. I have been in too many rehearsals where I see kids with tuners but they dont even know they are at 442 or 444. And I rotate which players/sections use them so they are not on them all the time.