I bought my very first at a local feed store, Kahoots Feed & Pet. Some others I got at Tractor Supply, they have cheaper chicks than Kahoots. Kahoots definitely has more variety because of how the owner chooses them.
For my first 2 batches, I bought chicks from big r. However, I found that their birds were less healthy, and since then I buy from a hatchery in Wisconsin called Purely Poultry, which I highly recommend.