If I was knocked prone in the same square that I dropped my weapon, caused by Stunning Fist, is retrieving my weapon still considered a Move action?
Yes. There is nothing in the rules to change that. You're still retrieving a weapon that isn't on your person.
Thank you!
Also worth noting that drawing the weapon from your belt scabbard/sheath is a move action, even if it's a little bitty dagger. Or picking up off a table or weapon rack next to you.
Remember that "Move Action" does not mean you are moving. They used to call it a "Move-Equivalent Action" which made it more clear that this is an action that takes some time and some of your attention, about as much as moving, but that it doesn't really mean that you are, in fact, moving at all.
So regardless of whether you must bend down to pick an item up off the floor, or merely reach to a nearby table or to your belt to pick up an item there, or if you're prone and merely reaching to pick up an item nearby, all of these are "Move Actions" that fall under the heading of "Manipulate an Item":
This includes retrieving or putting away a stored item, picking up an item, moving a heavy object, and opening a door. Examples of this kind of action, along with whether they incur an attack of opportunity, are given in Table: Actions in Combat.