Istanbul has for a long time been divided into districts & each district keeps the birth records of its residents. So, if you don't know where a person may have lived, it is very difficult to locate records, especially old ones. Records from the 18th century may not exist anymore & even if they did, they would not be easily accessible (they may be at the archives somewhere and may require special permits to access). Moreover, they would be in the Arabic script & unlikely to be indexed in an orderly fashion.