Where in NZ are you?
Theres a place in Manukau off Great South road that has a lot of 2nd hand parts.
Its opposite the large Caltex, just down from the new Motorway on ramp at the end of Te Irirangi(sp) drive.
Otherwise your best bet is your nearest BMW dealer.
However you won't find much that will make your 320i faster, other than either a Turbo or SC from the states or Europe, or a 328i or M3 engine from a wreck. (see above)