A 7mm allen key is rare indeed. I was surprised to need mine this week. I have a Philippe stem that requires it. I started to panic, because it took me a while to find my 7mm.

I suppose if you don't have any luck, you could take an 8mm allen key and very carefully and patiently file it down. I've had to do this with nuts once or twice, and the results were barely acceptable, but I had little choice.