I’ve experimented a fair bit and now get my beans (mostly from Starbucks) ground to a number 3 which I believe is an espresso grind. Most of the time this is perfect but grinds can vary from machine to machine, freshness of the beans, humidity etc. Occasionally an espresso grind has been a little to fine. I believe that most places know what an Aeropress is and will have a recommended grind slightly coarser than espresso grind. 
When sailing or camping and time slows down I grind my own coffee using a Rhinowares grinder that fits perfectly inside my Aeropress. Something very therapeutic about hand grinding your own coffee.
