Unless you're there at park open, don't expect to get this pin at the park. And Downtown Disney will likely be the most crowded location as that's where everyone who doesn't have a pass will go. So if you can go before work (and have time to wait in the line), that's your safest bet.
While the past few Windows of Evil have lasted as much as a day or two (the Queen of Hearts sold out Friday, while Kaa and the Horned King lasted a few days each due to lack of popularity of the characters and the series slowing down), this is going to be a hot series, plus a popular character and I would expect it to be sold out within an hour or two of park open. I could be wrong, but that's my expectation, so my recommendation is if you can go first thing, that's your best chance at retail if this one is a high priority for you.