Last year, when I bought farm store chicks instead of ordering from the hatchery directly, I made a point of looking up what the chicks of the breeds I wanted were supposed to look like.
If your feed store keeps the chicks in open-topped bins rather than closed cages it's not necessarily the store's fault when they get mixed.
At my local feed store I was hanging out watching chicks and saw one hop up to the top edge, balance there for a moment, then jump down into the next bin...