My understanding is that left overs are available to the general public on the day after full completion of the all parties. Not the day after each event, I mean when all 12 nights have occured. So this year, the last night is Dec 17th, so on Dec 18th you could go to the park and if there are any left they will be for sale to the public.
Leftover pins are available to party-go'ers at each party night.
These are in Expo Hall.
As an example, last year we were able to get pins for night we attended, plus pins from the 2 nights that preceeded ours. The earlier nights pins had been sold out already.