How do I reach the birth certificate of my grandfather who was born in Istanbul, turkey in 1899?
A good starting point is often the FamilySearch Research Wiki page for the place you are interested in.
Unfortunately, in this case, the page about civil registration for Turkey genealogy just states that civil registration in Turkey began in 1884 and doesn't provide any useful contact addresses.
This may be because, as it states on the main page, genealogy records are often kept on the local level in Turkey (Turkey is divided into 81 provinces). Perhaps not surprisingly, Istanbul is in Istanbul province (No. 34 on the map).
The website of the US Embassy in Ankarra advises that
You can obtain a family registry (nüfus kayıt örneği) from the Family Registry Office (Nüfus Müdürlüğü).
Following a brief Internet search, I think that the email address in Istanbul is:
Istanbul Governorship Additional Service Building 2 Hırka-i Şerif Neighborhood Adnan Menderes Bulvarı No: 64 Pk: 34093 (Provincial Directorate of Security) Fatih / İSTANBUL
(subject to the vagaries of Google translate!)
and their email address is: email@example.com (I would try this first).