After wanting one for a few years, I bought a Digital/DEC VT420 off eBay for £50. Obviously it would have been nicer if it was free, but it was a fair price to pay in my opinion. Unfortunately I don't have a keyboard yet, but hopefully I will be able to find one soon.
Here's a photo of the first time I switched it on:
But then I turned it off, and it wouldn't turn on again… After taking the whole thing apart I figured out it was the switch, as shown in the following video. Taking the terminal apart was a bit scary, but I was very careful to discharge the anode.
And here is a video of it starting up and performing its self test.
<2016-06-05 Sun 13:17> Update: I got a bit tired of reaching around to switch the terminal on and off at the wall. I took it apart once again and folded up a piece of paper into a ~0.8mm strip. I put this between the rocker switch and the thing which pushes it about. It seems to work quite nicely and the paper doesn't move around. Hopefully this will last a while.