I found my SMB after looking 3-4 times per day for about 6 months.
The best was tool was Search Tempest. Type in the word "Sportsmobile" and it will search every Craigslist in the country and ebay as well. The guy that owned it did post here, but he posted Craigslist first and I spoke to him first. By the end of the day he had 4-5 interested parties. He honored my first response, I paid for him to have the vehicle inspected by a ford dealership, the Ford Stuff, and we flew to see it in person a few days later. As BroncoHauler said, you must be ready to pull the trigger. I think it is a good idea to have a set of minimum requirements that you will settle for, as you do, and when the iron is hot, strike.
Also, depending on your location and your ability to travel to find one, check out FamVans.com, they usually have a ton of inventory. They are located in Fountain Valley, Between LA and San Diego in Southern California. Their inventory changes all the time and they buy bulk from big companies and messenger services, etc. Not a lot of low mile vans, but many I saw were in very good shape.
2005 5.4L E250 RB50 4x4 SMB
2000 4Runner Limited 4WD
But God demonstrates his own love for us in this: While we were still sinners, Christ died for us. Romans 5:8