Are you after a brand new set? Genuine BMW or after market?? The world is your lobster on this. I have found eBay good for this kind of thing. Just be very careful if buying genuine BMW to make sure they are what they say!!
Places to look for alloys;
Ebay
Forums...e46 forums e90 forums...any bmw forum
Gumtree
I found mine on e90post ..good fitment
If you have a good budget of say 1300 you can get a beautiful set of 313 performance alloys (inc tyres) which look absolutely amazing and pretty light weight