I agree with you about working with insurance agencies. Great way to not get paid for 90-120+ days. I learned the hard way early on. Either the company pays me directly or the person with the overspray on their car pays and gets reimbursed by the company or I don't do the work. Just not worth busting my butt working all day and then not get paid until the following quarter.
How much experience do you have with overspray removal? How about concrete? If you really haven't done much overspray removal, you might want to think twice about marketing directly because you may find it is very tedious work. Actually, I pretty much hate doing it. Pretty seasonal too, mostly spring work around here because the wind blows constantly from mid February to the end of May.
Why not advertise in one of their trade publications?