Where to get a copy of Power of Attorney document of a land ?

He should have submitted that POA to any public authorities like Munciipal Corporation, water Board or electricity dept for getting some approvals and this includes Registration offices.  Contact an advocate friend as to how that POA can be put into use with public authorities.

You can file RTI Application and seek certtified copy of your own power of attorney for less than Rs.42 ( Rs.10 RTI fee + Rs.22/- Registration + Rs.10/ copying fees @ Rs.2/- per page)

Just focus and apply your mind.  He must have obtained POA for some specific purpose for using with Public Authority.  You can certainly get a certified copy from them.

Search RTI Rules and regulations of that authority in locating Public Information officer, format of application and on payment of RTI fee.

This is simple and don't worry on this aspect.

If it is a Registered Power of Attorney, you can immediately obtain such certified copy from Office of the Registrar,Old court building , kashmere gate, delhi). by paying less than Rs.100/

Apart from this, you can at any time cancel the POA given to others under law and may publish it for the use of Public in news papers as abundant precaution.

POA - Were you the principal or the agent? The facts, as posted, lacks clarity.

Mr. still you have not answered whether it is registered or not.  However as you are one of the Agent of POA, you can get the copy from the Principal of POA.  and the Principal of the POA may cancel the same during the period of 30 years.  so verify in Sub Registrar office where the POA was registered.  and apply to the sub registrar concerned for certified copy of POA.