I had lost a wheel cap back in 2004 (I remember the year because it was our first trip in our new Eagle 5W). I pulled a cap off one from a wheel and stamped on the inside of the cap was a name and a number which I googled. This lead me to the manufacturer which had online purchases of that exact cap. You could try that looking on the inside of one of your existing caps for any information which would allow you to track down a replacement.
'06 2500 Silverado 4X4, Duramax LBZ Diesel, Firestone air bags
'08 Tracker Pro Guide V-16 Boat
'12 Pinnacle 31RLTS