The builder would keep the original agreement entered into and duly cancelled with previous buyer

If it is with registrar office take search in the records of sub registrar

2)obtain a letter from

Builder that said document is not in his possession and has been lodged in sub registrar office

3) no need to file FIR

If a registered document is cancelled by executing a registered cancellation deed, then the original document shall be held in the deposit of the registration department.

The original cancellation deed shall be in the possession of the executor of the said cancellation deed.

Builder says it is with registar office,i want to get mortgage loan on it and bank is saying i need to produce original sales deed for first purchase

If the builder states tht the same is with the registrar's office, then you may apply to get the certified registration copy of the same and produce it before the bank, because the cancelled registered document shall be deposited with the registrar's office at the time of cancellation.

Same bank gave me loan first time but for mortgage they are asking for previous sales deed,should they have done this first time only when i applied for loan with them?

You have to comply with the requirement of the bank for granting the loan

in this case can i file FIR against Bank or Builder?

Not maintainable and even the police will not entertain any such case or complaint made out of frustrations.

Ideally it should be in the office of sub-registrar but anything is possible in this country. You can conduct a title search in the office of sub-registrar to find the sale deed and if you are able to trace it then a certified copy of it can be applied. No FIR is maintainable for this.