Thanks, Patrincia for that link!
I often rely on the small packs of edible flowers sold at the grocery and specialty stores ~ you can also get them at the farmer’s markets sometimes here in the bay area and I am sure elsewhere too.
Besides, what you buy in the store, nasturtiums (which you can find growing wild many places) are brightly colored and edible. Just be careful to rinse them off if they are near a road. They have a peppery flavor.
Other than that http://www.gardenguides.com/plants/info/flowers/annuals/pansy.asp are beautiful and easy to grow yourself if you want to create your own supply. A friend did a wedding cake with pansies from her own yard. She picked and crystallized them as they bloomed over a couple months and had a beautiful cake, with panies all around and trailing down the side. (It’s also great for beautifully covering any flaws in the frosting you are concerned about!!)
The pansies in the pictures on my flickr page were given too heavy of a coat of egg white wash so the sugar is a bit uneven. If you don’t plan on saving them and can use them the same day, you can just put the edible flowers on “naked” which I thought was prettier in the case of some of the flowers there.