I was looking at
http://www.circlebmw.com/parts/rim.htm for new OEM wheels (single or set) today. But they are not cheap (no surprise). Others like morristownbwm.com also. Looks like there are some differences in prices, so look around.
You can check ebay to see if anyone is selling just the one wheel used. If you are lucky, you might find one, and if you are REALLY lucky, it will be local so you don't have to deal with shipping. There is a forum for items-for-sale on here, too.
There are a number of new replicas for sale on ebay if you want to take a chance on one of those.
As far as mixing and matching replicas/OEMs; I wouldn't personally do it (I like to keep things uniform is all I can say). Can't give you "technical" reasons not to.
TTM