I've bootstrapped consulting work by turning my current gig into a consulting gig -- I decide to leave a company, but they really can't find one (or two or three) people to replace me in two weeks, so I volunteer to consult for them to fill the gap.

I then charge double and work half while they drag their feet for 3 months trying to hire a replacement -- by then, I've had time to line up another gig and bank some cash. If you're nice about it and do a clean handoff to the next person, everyone is happy -- they don't even generally care about the money, they care about continuity of service.

Not doable for everyone, but it's worked out for me a couple times.