your wheels are the American Racing Fuel series. I have a similar set in black. They take a snap in dome cap. Unfortunately so far I can only find the chrome covered plastic cap. The good news is they are around $15 each so even if you had to replace them once a year or every couple of years it's no biggie. I will do some more looking and see if I can find a stainless version. Here is a link:
American Racing Center Caps - summitracing.com