I've heard numerous times that the United States produced a ton of Purple Heart Awards in anticipation for the invasion of Japan. So many in fact, that the US hasn't needed to produce any more stock since.
Is there any truth to this story? If not, how did it it get started and if it is true, is there just a warehouse full of Purple Hearts somewhere?
it used to be true (real hearts for Japanese invasion) but it's not anymore (created new ones). It's a bit complicated but we started printing new purple hearts around the year 2000. I don't know who started this story but i assume it was true when the anecdote was first told.
is there just a warehouse full of Purple Hearts somewhere?
Philly. And it makes sense. We need big government warehouses because the military has a lot of stuff it needs to store and it's not like the DSCP just holds purple hearts it's much more general military warehouse. Here's how it describes itself http://benefits.military.com/misc/installations/Base_Content.jsp?id=4125 . As the link shows it's unfortunately much less exciting than your question suggests.
as a sidenote: stars and stripes seems like a good source if you have more questions broadly similar to this. As a newspaper focusing on the US military it's probably going to have covered any major scopes type question on the topic.