There really is no substitute for doubanjiang in mapo tofu. I guess there is a Japanese version of doubanjiang that is a bit less spicy, and Lee Kum Kee makes a version as well (its labeled as Toban Djan), but I would just mail order some of the real thing made in Pixian such as in the link @biondanonima shared, if you can’t find a source locally. I’m in San Francisco and a few Asian markets stock it here.
I got a package of this from Mala Market a few months ago but haven’t tried it yet (looks like its out of stock at the moment):
