Looking at completed listings on Ebay, the caps you are looking for seem to show up all the time. There are only a couple I see listed right now:
http://www.ebay.com/itm/VINTAGE-60S-...-/111519221035
http://www.ebay.com/itm/CHEVY-PICKUP...-/191416076489
If you are patient, you will find a nice set on the cheap. Try these searches:
"(hubcap,hub cap,hubcaps,hub caps,wheelcover,wheel cover,wheel covers,wheelcovers) (Chevy,Chevrolet) (bowtie,bow tie)"
Or
"(hubcap,hub cap,hubcaps,hub caps,wheelcover,wheel cover,wheel covers,wheelcovers) (Chevy,Chevrolet) (truck,pickup,pick up,van)"
Jason