Unfortunately it's unobtainable legally. Of course you can find it on some abandonware sites if that's not a problem for you.
GOG is working to get LucasArts (now Disney) on board, so if we're lucky we might see it here someday, along with lots of great games made by LucasArts.

You can find it on ebay and that's all. We're all waiting for it to be available online...

I have played it. Is awesome. Still have the CD I can play using ScummVM.
I don't think it is being sold digitally right now, unfortunately. Best you can do is search on ebay or amazon.
Might come on steam at some point, since they have Indiana Jones and the Fate of Atlantis and The Dig.

It is in the hands of Disney. At this moment only one pack of classic Lucas adventures has been released and that is through Steam. Sadly Day of the Tentacle is not among those released games, only Indy games, Loom and the Dig.

I would say that your only chance to get a legal copy at the moment is to buy an old retail version via stores like eBay or Amazon.

Day of the Tentacle is my favourite adventure game of all time, so I highly recommend it, obviously. You could buy an old retail version, yes or download it anywhere (there are many places and all of them are illegal of course).
I am waiting eagerly for them to arrive here in GOG.com, in twenty years, or maybe thirty.
But hey, you can vote for it in the wishlist at least.