Non-employee collaboration on negotiation evaluations

Summary

What are some options for non-employees from other companies assisting with evaluating supplier negotiations, other than setting them up as contingent workers?

Content

We are public sector (County govt), so we provide services to the community.  If there is a supplier negotiation to request proposals from community service providers on the services they can help us provide, other people external to our organization need to be involved in evaluating these proposals.  For example a City that would also benefit from the services, or people who may have used these types of services in the past.  There may be cases where individuals from our supplier base might need to help evaluate proposals from other suppliers.