There have been several threads on this in the past, but I think the OEM stuff you can only find at the dealer or online (like at turnermotorsports).
I found a dealer that had a pretty fair price in my area and I bought 3 bottles to have on hand.
A word of advice, when you refill do it in at most 500cc increments, then recheck your level. My car told me I was 1 quart low, and when I filled it with 500cc and rechecked I was at the max. If only we had a dipstick...
"This is like having a house-cat thatís a bengal tiger." - Todd Deeken, everydaydriver.com, on owning an e90 m3.