I'm born in HK also and I left went to the US when I was 7. I didn't have a HK PIC. If you are not in a hurry and are not going to HK soon, you can apply for a HK ID at one of China's Embassy at DC, Chicago, Houston, LA, NYC, and San Fran.
Are you born in a hospital or nursing home? If so, that is the only way to get a cerified birth certificate that HK will acknowledge as legitimitate. If you are born at home by a midwife or something and is certificate by the midwife might not be as concrete proof as a certified birth certificate. If not, it might be harder for you to apply for one.
For me, I had to apply for a passport as well as for a HK PIC. I needed about $90 in fees and 5 self portraits to start in initial application process, and this can only be done in person. Afterwards, they mailed me a letter requesting more documentation including a copy of my HK birth certificate, my US naturalization certificate, copy of my us passport, documentation of when I permanently left HK and other things. Afterwards, they might or might not ask for more documentation from you (they only request for copies and not originals) and this is done by mail.
The thing is that if you have more documentation of who you are like a copy of your expired green card, school records, etc... might make the process faster. For me, I applied for my HK PIC and passport as you can't just apply for just the HK PIC alone and I started this January and it is still going because there's some discrepency on my US passport that I am trying to straighten out. Hopefully, they got everything and I will have to go the Chinese Embassy one more time to get my picture taken for my passport and HK PIC...
Like I have mentioned, if you have the patience and will not be going to HK soon, you can apply by mail, but you will have to go to the Embassy to submit the application and to pick up your passport and it will take months. Otherwise, if you are visiting HK soon, you can go to Wanchai to apply and it will be much quicker and you don't have to apply for a HK passport also. Good luck.