I've seen various hybrids of California poppy in pastel colors including creamy yellow. Most of the seed catalogs carry a few varieties, usually in mixes, but I believe the ones marketed as cream or near white are probably what you saw. There are also some pretty pastel yellow Iceland poppies that I've run across in mixed seed too.
I just purchased some California poppy seeds that I think may be what you are looking for. They are called White Linen (Eschscholzia Californica) it is described as a garden variety of the California Native. I picked up the bag (not just a packet) of seeds at our local Home Depot, but I live in California so that may make them more available. The seeds are from the Stover Seed Company out of Los Angeles, CA. http://www.stoverseed.com/flowers.html
I just placed the seeds in the ground about a week ago so I have no idea how the flowers actually look. The picture on the package is a beautiful pale yellow poppy! I hope this helps out with your search.