Though the website of DOPT does not show the postal address of the CPIOs but it clearly indicates their telephone numbers where you can call them and have their complete postal address. I am providing a link to the website for your convenience.
Yes, I do agree that the organisations are supposed to provide such info in their website. A lot more information is supposed to be provided by these organisations but they don't. Thats why RTI Act is there. You can include this question with your other questions while sending your application to the DOPT.