Does anyone know the best source for Guillot roses in the U.S.? I'm talking mail order, but if someone is in CA and knows a good retailer, please let me know.
The House of Guillot has been breeding roses for well over a century. The answer to your question depends on the specific Guillot roses you're searching for. The modern varieties in RU's Guillot Collection are fairly recent introductions. Most (if not all) of those are members of a group that Guillot markets under the class name 'Generosa'.
RU sells numerous roses bred by various members of the Guillot family -- roses that are not included in the collection that Jaxindel linked to above. The Guillots hybridized many famous old garden roses -- Teas, Chinas, HPs, etc. There are several reputable nurseries that carry many of the antique varieties. Info on the specific roses you want would be helpful in guiding you to suppliers.