I'm sorry to hear of this.
When an attorney dies, there are a couple of ways to access the client files.
One way is to contact an associate of the firm, if there are partners.
Another is to find out who the personal representative of the estate is, as they will have access and control of all pertinent assets.
The other is to attempt to locate the receptionist.
One can find out who the trustee is by checking with the state's secretary to see if the business was registered; if so who the registered agent is; or by checking the ownership of the property, if the law firm owned the property.
If the lawyer had a will, there would be a probate estate; the executor would file in the county where the attorney resided.
Probate/surrogate records are a matter of public record, so the public can access this information.
As for obtaining a copy of a power of attorney, any bank or financial institution that was provided with a copy will still have it on file in most instances; it can be requested that they provide a copy to the agent or the principal.