Business ID document
Most small town Chamber of Commerces have a low cost "individual" or "home-based" membership, which conveniently does not allow listing in their directory or business promotion, and at a very reasonable cost. They will issue a Chamber membership card to you, which can be used as ID.
When joining a Chamber, a business card will suffice (and you can get these printed up over the internet with whatever vague information and your ghost address you want to use). If asked, you can give the Chamber some vague information, such as "I am anticipating doing some contract seminar work on privacy issues in the area, and thought it would be a good idea to join the Chamber.
Many times, Costco or Sam's will also be a member of their neighboring Chamber of Commerces to promote their business, and you can use their local representative to even get your membership expedited, with a friendly call in advance to them from the Chamber office.