Not only does the whole idea of adults wearing onesies make no sense to me, I’m at a complete loss as to why you’d want to wear one that’s even dorkier than a stock onesie. But that didn’t stop someone over at the BBC from licensing the TARDIS and the Doctor Who logo for just that purpose.
According to the product description, the TARDIS onesie is “comfortable, warm and snuggly” – which I suppose is good, because I don’t think anyone else would want to be seen with you wearing this, and you’ll have only your onesie to keep you warm.
If you absolutely must have everything made with a TARDIS on it, you can get the onesie over at Red5 for £39.95 (~$62 USD). I suppose if it could actually transport you through time and space, I might wear one. Or not.