The off chance of finding this will be a random back woods store that has NG and has no idea what they are sitting on, the distributors tried to bring most of it to stores that were selling it. My store has been out for at least 4 months. That being said there is always that chance. (Oh it was such a good beer too!)
Someone sent me about 3x 4-packs in March. I'm not sure if it was their personal stash or they were able to grab it off the shelf.
I found it at a hole in the wall store outside of Madison in March, they had tons so you could possibly still get lucky
I stopped in to a couple spots last week (Woodmann's in Menomanee Falls and a few places in Fond du Lac) and i didn't see it. In fact, the only Thumbprint stuff I saw was Cherry Stout. Sad face.
This helpful tool on the NG website is usually pretty accurate: http://www.newglarusbrewing.com/index.cfm/beers/BeerFinder
There's a store in Milwaukee that still has it available...I just bought a 4-pk a week or two ago and had one in a fruit beer tasting. It still tasted great to me - let me know if you're really looking for it and we can figure out a trade or something!