A cute red squirrel carrying an acorn.
Where you can buy online
TeePublic (US and UK)
RedBubble (US)
Threadless (US)
NeatoShop (US)
You can find the design on t-shirts in most stores, but also on other products depending on the store (e.g. hoodies, mugs, stickers, notebooks, phone cases, greeting cards, cushions, socks etc).