If it's high margin/profitability (50%) and you don't require many clients to sustain your business - do it yourself. Build trust. Only make your clients pay after execution. No prepayment. Get more volume from those clients while slowly but surely going after other clients. Favour trust, and quality over quantity and volume. Too many B2B providers give all they have at the start to impress new clients/prospects but then drop the ball when they scale too rapidly. It's a snowball effect and then you lose business (and reputation), and you are forced to keep finding new clients till your business collapse due to poor servicing.
If it's low margin or high volume I don't know what it takes to find clients so couldn't help from this front (ie advertising, lead generation etc)