I thought there was one in Ullapool but I can't find it on the Shell finder. I *know* there is a normal Shell garage up in Thurso, but I can't find that either....
There is a normal shell station in Fort William as well, it doesn't show up but there is no optimax either. Neither of the garages in Onich/Glencoe have it.
I think all of the petrol for Caithness and Sutherland comes in at Scrabster (Thurso) by boat, so if one of the Shell garages up there has it, they probably all will.
I used to have 4 shell garages fairly handy (n. ayshire) and all changed hands within a year - arse!! so shell can stick their optimax and considerable other company car fuel sales....
If you need Optimax, you're better off stocking up with some octane booster and just filling up with normal unleaded wherever you can find it. Its hard enough to find any super-unleaded of any sort...