Hi fellow newbie, I asked a similar question and it seems that it hasn't yet been done to the 18i.
The clever money seemed to suggest this place
http://www.evolveautomotive.com/evolve-ecu-remap-performance-upgrade-bmw-e89-z4-18i.html
I'm still in 2 minds, I have had a squeaky bum moment as standard which seemed quick enough for me.