I just finished the game and I'm missing three battle stamps on the bottom row. I've bought all the available battle stamps and I still have no idea where to find the rest.

The very last one is given to you by defeating the witch at the end of the game ( well almost the end ) as for the rest they show up at the store eventually 

Do other stamps show up based on your level or am I missing something? 

I think they may show up based on how much candy you have on you. I know the last few didn't immediately show up for me, until I had enough candy to afford them (although it may have been something else that triggered that, I can't say for sure)

Each time I leveled up, more stamps became available.  Are you @ level 10?

Leveling up, and do you have the larger candy bags from finding the hide and seek kids?

You also get one for collecting all the costumes too I think.