The caste certificate should be issued by the competent authorities only . Following is list of some such competent authorities -
1. District Magistrate/Additional Magistrate/Collector/Deputy Commissioner/Additional Deputy Commissioner
2. Revenue Officer not below the rank of Tehsildar
3. Sub-Divisional Officer of the area where the candidate and/ or his family resides
The nomenclature /designation of competent authorities may vary in various states.
The website http://punjab.gov.in/caste-certificate-obc caters to the need of residents of Punjab in such matters.
Regarding acceptability of such certificates issued by the competent authorities in case of any particular examination, it is better to seek clarification directly from the examination conducting body to be on safer side as legally caste certificates issued by the competent authorities should be treated as valid.