Vala - Yes, this is an excellent tip as well!

Even though I still think - given my past experience - I will
try to adhere to the 'buy it right when I see it or risk never finding it again' philosophy, what you suggested is very important.
I've often thought of this before - clearly, the shops right along Main Street are going to run out of things more quickly, as those are the ones that everyone passes by when either arriving or departing. Everyone will descend upon the Emporium because it's big and they will assume that it has everything they could possibly want - sort of like the hub of all souvenirs. Sometimes the 'smaller' Main Street shops get overlooked - for example, the China Closet often has things that have sold out in other stores, and it is often less crowded.
Also, people will head to World of Disney in DTD - again, because it's big (enormous, actually), and they assume it will have everything under the sun.
But, let's say that even the China Closet and the other shops along MS have sold out of whatever the coveted item is that you want.
Think outside of the box. Forget where you originally saw the item (where it is now sold out). Look in the out-of-the-way shops. Even if it's not a Western-themed item, look in the Frontierland shops. Look in the Fantasyland shops. Look in the tiny, hidden, tucked-in-an-alley shops in New Orleans Square. Look everywhere. Look in all the lands.
Apply the same outlook to the shops in DCA - you may not expect to see an item that you found in DL to be in DCA too, but it very well may be!!
And even though the Gag Factory in ToonTown is usually crowded, I
think it mainly tends to be crowded with the people who were already in TownTown anyway, but I don't think that too many folks who had no intention of going to ToonTown in the first place are packing into the Gag Factory just to shop.
However, the Gag Factory usually has a good supply of things and they don't sell out that quickly. So even though you might have to fight off the masses that piled out of Mickey's house to search the Gag Factory shelves, it just might be worth a trip into ToonTown to find that item that sold out on Main Street or in World of Disney.