The first thing I would do is find out why SRC has chosen to stop assisting with leases and insurance. Perhaps they could continue to do so with financial and administrative assistance from The Green Institute without having to re-invent the wheel.
Liability insurance usually has a minimum policy premium which will include a substantial amount of land, which is why they were able to keep the premium per garden down. But perhaps they should have been charging more per garden to enable them to continue this service.
Whoever obtains the insurance needs to have signed the lease. You can't insurance something without 'an interest in it' (translates=lease).
Unless SRC was already paying owners for land, don't start now. Most groups I am aware of sign 'fee waived' leases for community garden land. Why? Because the landowner is gaining community good will and is saving the $ that would have otherwise been spent keeping the vacant land clear of weeds and debris (a legal obligation in every jurisdiction).
If gardens have dues for members, there isn't much reason why they cannot cover $150-200/year for the insurance.
But again, start with SRC to see if their assistance could be put back into place. Then learn all about their experience, their insurance company, etc. If they were satisfied with their insurance company be in touch to learn the particulars of the policy. For comparison, be in touch with the Land Trust Alliance to find out what agency offers the policy that they invented for their members. You don't have to be a member to obtain the coverage.
Good luck and let the list know what you learn.