A free permit is required from the Forest Service, even if you are only planning on harvesting a small amount for your personal use. My best advice is to ask the rangers when you stop in the Ranger Station to get your permit. I usually go to the Zig Zag Ranger station to do this. All that is required is a valid photo ID.
Be aware that there are some areas off limits to picking, as they are reserved to Warm Springs tribal members due to treaty rights. These areas are generally well marked. The rangers can also help you with this, and even provide maps.
If you aren’t interested in picking, there is generally a stand in the parking lot next to Charburger in Cascade Locks that sells them during the season. You may also be interested in the Mount Hood Huckleberry Festival.
However, my favorite place to find huckleberries is in one of the famous milkshakes from the Huckleberry Inn in Government Camp.