I knew that eventually the chicks I raised indoors were going to live outside, and I'd heard so many horror stories about chicks (even on medicated food sometimes) ending up with coccidiosis. So I took the advice of folks who had been doing this a lot longer than I had and tossed a big clump of sod, right out of the garden, into the brooder when they were about 4 days old.
This did multiple things - it gave them something to do besides running from one end of the brooder to the other over and over again. It distracted them from feather and eyeball pecking on their brooder mates. It gave them the opportunity to ingest some small sand particles as grit, and if they wanted to nibble on the roots in the sod or found a worm, that was a bonus! And what else is in the soil? Trace minerals they don't usually get from a bag of food! But most important to me, it exposed them early on to what they'd be living on once they went out. We tend to keep them in these clean little boxes, exposed to absolutely nothing, then expect them to thrive when they go out and have to face all the pathogens that are in the soil, the water (and they'll drink from any source!) and the air. Why do we do that? A broody has her chicks on the same floor or soil she's living on from the start, and they scratch and dig in it, and eat it. They do fine.
The fun thing to do is put the sod in there and watch them. First they hide - there's suddenly a big monster in their brooder and it's going to eat them alive!! Then a couple of the brave ones will go over and investigate. They'll peck at it, get on top of it and scratch, and the next thing you know it's a free-for-all in there! Digging, pecking, scratching, and when it's all broken down they love to dust bathe in it. It's just plain good for them all the way around.
So supplements? If I could name only one thing that's as close to a perfect supplement as possible it would be that simple clump of dirt!
Edited to add: Like @azygous I also brood my chicks outdoors now from the start. They do great!! So I don't rely quite as much on the sod, but if they are getting restless I still toss one in there.