You have to know what you are looking for before you know where to look. A Will can be handwritten by the decedent. A Will can be downloaded from the internet or bought at a stationery store and filled out by the testator. A Will can be drafted by a lawyer, or maybe someone else. Moreover, it may have been many years since the Will was created, undergoing various amendments or changes on the same or different pieces of paper.
Wills are usually kept by the testator.
There is no central repository for Wills. While a few states have a registry for Wills, very few Wills ever get there and the likelihood of finding what you are looking for are slim. Most Wills are kept by the testator. The testator will often keep the Will in a safe deposit box, or in a drawer at home, and/or give a copy to a friend or relative, each of which presents its own set of problems.
Lawyers may not be the best source for finding a Will they drafted, even if you can find the lawyer.
Unless it is a very small town and/or there is some knowledge who the lawyer was, there would be almost no way of finding out which lawyer drafted whose Will. Lawyers are not required to keep original Wills and rarely do, they are not required but sometimes do keep copies of Wills, and depending on how long ago the Will was made and whether or not the lawyer is still around, it becomes a major challenge to try to find the lawyer and then find whether there can be found a copy of the Will you are looking for. Sometimes publishing a notice in a newspaper which may possibly be read by local lawyers inquiring if they have grandma's Will might produce a result, but that would be rare.
Wills get lost or destroyed under suspicious circumstances.
Sometimes the loss or destruction of a Will is a good thing -- for the person who stands to gain, either by intestacy or based on an earlier Will. Let us suppose that mother disinherited that lout of a son of hers and favored her other children. If that son is the first one to find the Will, we will never know. This, of course, highlights the importance of placing a Will out of reach of people who would benefit from its loss or destruction.
Wills are withheld or kept hidden by someone for ulterior motives.
Sometimes the person in possession of a Will known to exist refuses to show or make a copy available to other interested persons, heirs, beneficiaries, maybe to vex the others, maybe from a misguided sense of power or authority, or who knows why. Or, perhaps, there are no assets subject to probate (such as joint tenancy or community property) so there is no point in doing anything with the Will at all.
The State or Court will not help you find and file any Will
Sometimes we are asked "why won't the State (or Court)" find out whether there is a Will and/or what happened to it and/or what will happen to the decedent's assets? It just doesn't work that way. The "interested person", heir or beneficiary, must initiate all the investigation and court proceedings, seeking specific relief which the court is empowered to grant. Meaning, if you are the person effected, you will need to hire investigators, lawyers, and initiate and pursue all court remedies necessary to obtain any result you seek. In other words, don't wait for the government to do your work for you because it probably won't happen.
I know he/she has the Will, how can I get it from him/her?
Since it involves time, money, effort, and probably a lawyer, you have to be relatively confident that valuable assets are at stake and you would otherwise be entitled to some significant portion if the Will is made available. We assume here that you are relatively certain someone has the Will and will not reveal it or otherwise holds on to it. At that point you would employ a probate attorney in the county where the decedent last resided, and file a petition to be appointed as administrator of the estate. Upon receiving notice you may be relatively certain that whoever is holding on to the Will would be expected to come forward with the Will if it in any way benefits him or her. If not, then the estate proceeds as if no Will exists and the laws of intestate succession (children, spouse, parents, siblings, etc.) would apply.
There can be penalties for not timely filing a Will with the appropriate court.
Most states have laws requiring that an original Will be filed with the appropriate court within 30 or 60 or a specified number of days following the testator's death. Assuming that it can be shown that a valid Will actually exists and that a specific person has that Will, they can be penalized or held liable for damages for willful failure or refusal to timely file that Will. Once again, the time, money and effort necessary to enforce these laws would be pointless if there are really no substantial assets subject to administration and/or the interested person would not stand to benefit in any regard.
Conclusion -- Keep Wills in a Safe Place
"Safe" means safe from someone who would benefit from the loss or destruction of the Will. Putting it in a safe deposit box would not be safe if the key is found by the omitted or disinherited heir (or his/her mother in law) in the decedent's nightstand drawer before the other relatives can find it. Putting it in the hands of a trusted and known friend or professional who could not benefit from its loss or with ties to someone who could, and copies to alternate executors, would help decrease the incidence of disappearing Wills.