This post of nycindie's has several links to resources to find a poly-friendly therapist. I don't think a therapist who isn't poly-friendly would be very helpful to someone who is or is trying to be in a poly relationship. Having said that, though, I might ask your current therapist outright if she's open to the idea of poly, instead of trying to infer. You deserve a therapist that's actually going to help you work on what's best FOR YOU, so requiring a therapist to be poly-friendly is in no way looking to be placated. It's looking for help that fits your needs, which is generally necessary.
Pan Female, Hinge in a V between my mono (straight) husband, Monochrome and my poly (pan) partner, ThatGuyInBlack