Work it the same way as used cars!
You have a approved lender, you quote the job, you arrange the finance application and submit it, approved lender sends you the funds, you complete the job, finance company gives you a fee for suppling the finance on their behalf.
So try approaching some of the premium lenders, premium = reputable well known lenders with sensible rates, IE black horse etc etc for example.
See if the services you offer are the kind of thing that they would be interested in financing, and if it is then you need to negotiate the fees that they will provide to you for bring the customers to them.
So this way everything is managed in house by you, you are in control of applications and time frames and you make money on the finance.
This saves pricing a job, risking having it undercut or gazumped while the client shops around for finance and avoids the waiting game while you wait for the finances to come from a unknown party. if this was the case you would have to wait till payment was cleared befor commencing work as opposed to using your own finace channels where you could commence as soon as the confirmation is emailed to you in the knowlege that payment is a g'tee to arrive.