The will and an application for probate must be filed in the probate court for the district where your father resided in order for your step-mother (or anyone else) to be appointed as executor. Once filed, the will becomes a public document and you will be entitled to a copy. Note that there are some matters which do not require formal probate, such as those where the decedent re-titled all of his assets into a revocable trust prior to his passing. Since your step-mother is "hiding the ball" here, your best bet is to retain an attorney to represent your interests. Good luck to you.
This information is presented as a public service. It should not be construed to be formal legal advice nor considered to be the formation of a lawyer/client relationship. I am licensed in Connecticut and New York and my answers are based upon the law in those jurisdictions. My answer to any specific question would likely be different if I were to review a client's file and have the opportunity to interview the client. Accordingly, I strongly urge you to retain an attorney in your jurisdiction with respect to any legal matter.
If she does not produce the will I would recommend that you discuss your best options with an estate attorney. Usually a letter from an attorney will cause the party to produce the will. If they still do not produce the wil your attorney would prepare a probate petition to name your the administrator and compell her to produce the will.