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The form you need is available at http://www.irs.gov/ . The form number is XXXXX Here is some additional information to help you get this correct the very first time.
Topic 311 - Power of Attorney Information
You have the right to represent yourself or have someone represent you before the IRS in connection with a Federal tax matter. If you want someone to represent you before the IRS file Form 2848 (PDF), Power of Attorney and Declaration of Representative, with the IRS office where you want your representative to act for you. Your representative must be a person authorized to practice before the IRS. Your signature on Form 2848 allows the individual or individuals named to represent you before the IRS and to receive your tax information. Refer to Topic 312, if you want to allow a person to receive your tax information, but do not want this person to represent you before the IRS.
For additional information, see Form 2848 Instructions and Publication 947, Practice Before the IRS and Power of Attorney.
You should also note that NY State uses its own form so, if you want the individual to handle those matters as well you will need to use. This will also be acceptable to NYC if needed.