Have you read The Well Trained Mind? It might give you some general sense of where kids would be and most libraries carry it. I found I have to work above the levels recommended but the approach works well for advanced learners. I did not find MBTP to be on par for gifted children.
I have had some success with a boxed curriculum with history/lit. by using Sonlight above level and in a secular manner, but I only use the core, not science, or LA. I only think it is successful because it is literature based and I have avid readers. Otherwise you might want to stick with programs that allow you to do a placement test. I tend to order lots of catalogs and then put their lists together with the WTM and pick what would work well, this is my approach to younger level science and it's been quite successful.
