Krypton: Good comment!
Grumpytroll & others: The State civil & military aviation authorities are usually the ones ultimately responsible for obstacle data (ensuring it's captured, verified or appropriately qualified, and then distributing to third-party users such as providers of charts, digital terrain & obstacle databases, TAWS etc.
So it's most likely Garmin buys their data & updates from one of the 3rd party database prep organisations (eg, probably someone like Jeppesen) if not directly from the country's main civil aviation authority (like the FAA).
Assuming you're in the US, you should contact the FAA.
These sites will probably lead you to the right place or contact:
-- nfdc.faa.gov/tod/public/tod_faqs.html
From that you can select the Areas of Responsibility Map which will also give lead you to relevant contacts in your state.
-- oeaaa.faa.gov