Where can I find a release form to be signed by the beneficiary of an estate upon receiving the bequest ? I am the executrix.

You can usually find the release forms from the County Surrogates Office that issued the Letters Testamentary, and where the Will was probated.  Many of the offices have document available on-line and can be downloaded.  Follow the signature requirements carefully, and make sure the beneficiary's signature is witnessed by a notary if required.