1) tax payers who have made payment of bribe would not help divulge details of bribe paid by them . if possible obtain affidavits from them regarding bribe paid to check post officer
2) you can on basis of documents in your possession complain to the Lokayukt for inquiry if you feels that any public servant is guilty of an allegation described in the Madhya Pradesh Lokayukt Evam Up-Lokayukt Adhiniyam, 1981.
3) complaint on a plain paper is acceptable provided it contains adequate information to hold an enquiry. Such a complaint should invariably contain name and post held by the non-applicant, particulars of the allegations which are required to be enquired into and name and address of the complainant. A complaint may be presented to the Secretary to the Lokayukta or can be sent under a registered cover. The complaint should contain adequate and precise particulars to show that the case is covered within the expression 'allegation' as defined in Section 2 (b) of the Madhya Pradesh Lokayukt and Up-Lokayukt Act, 1981. The complainant is expected to verify facts and make correct averments after obtaining the relevant information by resorting to the provisions of the Right to Information Act and to annex with the complaint the relevant documents so obtained.
4) you can also complain to CM office regarding check post officer
5) do not know any lawyers specialising in anti corruption cases in MP