But it is not an art that is found anywhere and competent teachers are hard to come by, even in china or taiwan.
The other two commonly reffered to are tai chi chuan & xing yi chuan but they do not have the intricate footwork of baquazhang, which can be a great complement to your aikido; but in terms of internal energy development, it depends on the teacher. Some are obviously more knowledgeable and generous than others in this area which traditionally is reserved for advance students.
No matter what internal art you chose (they are all good) it ultimately depends on your committment to spend at least few hard years before you even begin to comprehend what it is all about when seen in the context of MA application.
For general health maintenance purposes or for alleviation of specific illnesses, that is another matter entirely.
Ask 'Baguabyrd' nicely for locations of Baquazhang schools in your area.