I recently purchased a set of 16" RSX wheels for the winter and I didn't want to use the Acura caps or the Honda caps from my summer wheels.
I found this link
Honda-Tech.com: Wheel and Tire: ► FAQ: Centercaps
and used a suggestion from there. Great link too. It shows alot of different wheels and whay caps fit them.
I purchased these from my local Lowes.

They are called Knockout Closure plugs. 2 in a package, found in the electrical supply area and they were about $2 for a package. They are 2" in diameter, which compared to the OEM caps, slightly smaller. They are metal (very shiny aluminum I think) and have small tabs on the backside similar to the OEM caps.
I did a test fit on my summer wheels and for them to fit without falling out, the tabs needed to be bent out slightly.
I scuffed the surface with some fine sand paper and painted them silver with VHT caliper paint.
Finished product.

I had to adjust the tabs slightly, but they fit perfect and have yet to fall out. I think they look much better then no caps at all.