I'm assuming that you're talking about the Alcoa aluminum wheels. Correct?
Front or rear caps?
I couldn't find one that was the correct diameter for my front so I bought one on ebay and enlarged the trim ring to accomedate it. Ended up opening up the hole about 1/8" all around. Other then it saying Alcoa instead of Airstream, you couldn't tell. I still have one on the road side that is OEM. The rears and TAGs are all OEM as well.
How are you taking the measurements? I measure across the bottom flange, and then the height. The problem I've found is that some have a wider flange then others and that throws the measurement off. I do have some calipers I can use to measure before the base of the flange that goes through the hole in the trim ring.