Commercial Project and can't get paid with NO BOND IN PLACE

[Bond Guy]

We are doing a commercial project and have had quite a few problems getting paid from our customer. Now, they are requiring us to get a bond before they will pay us for work that has been completed. There is nothing in our contract concerning bonds. How can they require us to get one now that we are almost done with the job, and how do we get bonded for work that is already completed. Please Help ASAP.