I've seen the official portal mug available from places like 'Think Geek'. However I wanted this to be something that might have actually been in the game.I spent a while searching the game for mugs, but I was disappointed to find that all the in game mugs just have jokes like 'Who Farted' on them.
My one is intentionally more dull. I've made myself an imaginary employee of Aperture. A quick search of the Portal 2 transcript showed that Cave mentions test associates several times, I also considered making myself the 'Party Associate' from the first game.
I've wanted to do this (and a bunch of other mugs) for a while now, but finally got it made with 'Vistaprint' .
It you join their mailing list and wait a day or two, they'll send you all sorts of free offers including a 'free' mug.
I think after postage and an additional cost for uploading my own image this one cost me £6.