I guess radio, newspapers and magazines mostly and at each salesroom. I do recall seeing a very early sticker, quite small, on the window, but I'm not so sure it was on a Model A, but it was from that same era.
Remember that Ford was a stickler that all Agencies charge the same amount for a new vehicle, -or the same amount for any parts or labor. This was in their contract. The only room the Agency salesmen had to negotiate on a price was by selling the customer accessory items to add to the end price.
Somewhere I have a copy of an era Ford contractual agreement with one of his Agencies. If my memory is correct, the Agency's commission was 15% on a vehicle sale and 25% on parts.