I would strongly recommend looking for a used 2013 MacBook Air (non-Retina). I used Ubuntu on one of those for a while. You should be able to get one reasonably priced directly from Apple at this point.
Only caveat here: the keyboard is obviously a Mac keyboard, which can be very irritating to those of us who like to have dedicated Home/Backspace keys.
Edit: another caveat. I just realized I had one of these for a while and stopped using it because the resolution is just too small. There's just not enough screen real estate to be truly useful without another monitor.
True, the resolution is relatively small. If you can live with that it is great hardware for the price and everything in at least 14.04 worked perfectly.
Edit: another caveat. I just realized I had one of these for a while and stopped using it because the resolution is just too small. There's just not enough screen real estate to be truly useful without another monitor.