My advice would be to take inventory of the businesses in relatively close proximity to your base of operation and try to ascertain the shipping needs of some of the small to medium sized companies. Being local is probably the most compelling value proposition you can bring to the table. Devise a marketing plan that emphasizes what differentiates you from competitors ... Knock on some doors, mail out some fliers, develop some personal relationships with warehouse / shipping managers, join a traffic or transportation club. I am a firm believer that the majority of small businesses prefer to work with other small businesses.
As to the load boards, the preponderance of the true expedite market is "owned" by the brand-name/well-established carriers. These entities primarily "exchange" capacity and loads through Sylectus, also commonly known as the "Alliance". Membership criteria is posted on their web-site (minimum fleet size reflects 5 trucks). DAT, Internet Truckstop, and others are really geared to the FTL sector, however, you can find some opportunities for smaller equipment. That said, finding anything worthwhile for your Sprinters will be a rare occurrence.