hi,

see you have to follow the rules and regulations of any particular exam and must follow the Information brochure of the particular exam, generally the income certificate can be issued from Block development office of the particular block,  Sub Divisional Officer in each District , Tahsildar Village Officers etc. however there is also in some of the states like odisha you may apply for e certificate. however follow the below link for apply,

http://edistrict.odisha.gov.in/