Firstly I have no trouble getting the Sherry mac 12 from 1st Choice so they are either clueless(most likely) or they don't have it in that store or area. No reason they couldn't order it in but I've found they don't know much about whisky's.
Secondly I'm not sure why you think it's legendary. I'm sure there are people on this site who love it and others like me who don't rate it at all. I mean check out how many times it gets mentioned when novices ask for recommedations, not very often.
All you can do is try it and find out for yourself.
As for Mac 10 haven't tried it.
Anyway you've now gotten a reply now, hope that helps as you sounded a little desperate.