Sara-Ann,
Hortico in Canada has Frederic Mistral grafted onto multiflora rootstock. I have a number of roses grafted on multiflora and I also have alkaline soil. Most of these roses do well for me, but I like to use soil acidifier on the soil around them, and once a year I use granular fertilizer for acid loving plants. I do this because multiflora roots supposedly prefer an acid soil. My Frederic Mistral was purchased from a very fine local nursery a number of years ago and is grafted on Dr Huey. FM is one of my favorite roses--huge plant, always in bloom with light pink, very fragrant blooms.
I would give Hortico a try; I've ordered several times from them and never had a problem. Diane

Diane - Thank you so much, I can't believe I missed that, I've been on their site before. Multiflora rootstock doesn't seem to be a problem at all for me, I have a few already and they are doing great. I better get it reserved ASAP!