Do you live in a metropolitan area, and if so, which? I know a number of places one can volunteer where I live, but if you are on the other side of the country, that wouldn't be much help.

Given that you have an anxiety disorder, you might look into volunteering with animal shelters--dogs are said to be calming.

Or, just walk into the vestibule of a church. In many cases there will be a bulletin board with a notice of volunteer opportunities, or a Sunday bulletin with the same. Note that this does not imply any religious commitment. Nobody's going to ask for your baptismal certificate if you're volunteering to sling hash for a dinner program.