BMW have confirmed that the E10 rollout is fine for Z3s however as you have a Z3M you should be using super unleaded anyway which is all E5 - most Tesco/Sainsburys supermarkets sell it, also Shell, BP, Esso etc.
Interesting have they guaranteed it in writing ??
Asking because someone on another motoring forum I frequent wrote to all major manufacturers asking them to guarantee E10 wouldnt do any damage to cars newer than 2002 - as yet not a single one has.
Also I'm guessing the OP is North America based, by their use of gasoline meaning the list of sellers wouldnt be useful. They do tend to have premium over there tho, thats only 5% which should be fine
As I've said before why wouldnt you use Premium E5 fuel, economy wise its around 10% more fuel efficent at least,. and gives better performance too, for only around 5% increase in price. All out local petrol stations have seen big increases in E5 sales since E10 was introduced