If the run is too small or becomes too organic in nature, it is possible they might not take in adequate grit. However, if they have access to your yard for an hour a day or more, they'll pick up all the stones and pebbles and broken chips of stones they possibly need. Chickens were kept, have been kept for centuries and until recently, which farmer ever considered going and buying grit? When I picture my grandparents the thought is almost humorous.
Sure, you want to pick some up at the store?, it'll do no harm. Our entire property is grit, so no, I don't buy any, except when I order a few tons of gravel for our driveway.