There are various contraptions out there that will turn a random door in your house into the TARDIS door — but judging from Amazon reviews and photos, this vinyl door sticker is the best. Created by Ukraine-based art studio Pulaton, it has a really nice level of detail.
Almost all the Amazon reviews for this TARDIS vinyl sticker are positive (except that one mentions a single typo in the "Pull to Open" box — hopefully that's been fixed.) The other option out there appears to be the Think Geek TARDIS Door Cling, which looks a bit more stylized and slightly more cartoony, plus according to customer reviews it's slightly harder to apply. Here's what that one looks like: