I just found that HarborFreight has a combined SAE/Metric hex wrench set which contains a 7mm.
http://www.harborfreight.com/25-piec...-set-5962.html
They are currently running a sale (or maybe it is a perpetual sale; I wouldn't know because I don't go there very often), so the entire set costs about $4 on their website. It's only slightly steep for a
good tool. I have no idea how good it is, but for a number of other sizes I'd be sunk if I ever lost one of mine, especially the tiny ones. So having some spares of all the rest helps justify the price for me.
Which is to say I now have a new set in my car. FWIW, the website description is slightly wrong. The nominal manufacturer is Pittsburgh, which is perhaps a brand name for HF, I dunno'. At least it has a wrench which
claims to be 7mm!