I have never sold to stores, but there is a book out called Flowers for Sale: Growing and Marketing Cut Flowers by Lee Sturdivant that covers this subject. I have a copy and it has been most helpful in growing flowers and also looking into opportunities should the occasion arise. It might be available at your library or local bookstore. Amazon sells it, too.

Almost forgot another one I had read: The Flower Farmer which has had great reviews, and also available at Amazon.

Here is a link that might be useful: Amazon

I never sold to a grocery store. Found it easier to take all my flowers to a wholesaler. They love fresh cut home grown flowers. They wanted same variety bundled in 10's without stems being cleaned of leaves. This was much easier for me than running around to various flower shops.