A pair of cute ninja penguins!
Where to buy online
TeePublic (US and UK)
NeatoShop (US)
CloudCity7 (UK)
PopupTee (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).