Yeah, they're tough to just go out and get...these are the main scenarios that come to mind.
1. You work at a company for awhile that transacts with the govt, you move to a position/responsibility that requires them to submit you. They pay.
2. You get a new job for a company, govt contractor, or govt agency that is dependent upon your background investigation being successful. They pay.
3. You get a govt job that doesn't require clearance and side-step/jump to a position that will get it. They pay.
Sometimes the foot-in-the-door approach is the most successful. However, CCIEs and CISSPs with substantial experience will have a much easier time getting a company to sponsor their clearance.