I give my horse a vitamin supoplement once a day everyday with his grain. I use equis vitamins and I've seen horses on horse guard too and they both work great! I also like the smart packs too, but I am down to one horse so it isnt a huge deal to buy a thing of vitamins.
Here is a link to the smart pack website: Welcome to SmartPak
This is what the horse guard is: Google Image Result for https://www.horseguard.com/shop/media/ccp0/cat/hg.jpg
You can't buy the smart packs in the store, but the cool thing is is they know when you are going to run out and automatically ship you more.
Horse guard and equis vitamins you can buy in feed stores.