Where can I find the current form for the release of claims and assignment of refunds rebates and credits?
First ensure that your cognizant Contracting Officer or Contract Specialist does not have agency-mandated forms. The FAR and DFARS does not contain mandated forms tied to your question. Don't forget to review applicable clauses to follow the direction for your contracts, such as FAR clause 52.212-4 (for commercial contracts) has a section about release of claims. If there are not agency-mandated forms, then the Defense Pricing and Contracting (DPC) website has an optional Final Release of Claims form that can be used for contract closeout, see below link.
Other civilian agencies (non-DoD) have Assignment of Refunds forms, so you can copy the content of those forms into your own document.