Popular at Brighton was the Pomona 1508 which I have seen before, but only found available in large quantities. I was told that Amazon have them, but they were out of stock when I looked. I found them on ebay, but selling for £16+. A correspondent from the Brighton meet wrote to say that there is a cheaper alternative, the Voltcraft ML-1 which sells for £7, looks pretty similar and is well worth buying, even with overseas postage. I have installed one on the desk and will try to rig up a second for travelling.
[Aug 2015] I return to the subject a few years later.