Candleberries are a brand-new item added to ARC Raiders as part of the Cold Snap update, and you’ll have to farm them as much as possible.
The item is tied to the Flickering Flames quest that’s part of the Cold Snap update. You’ll need to find Candleberries for the Candleberry Banquet. While you can obtain Candleberries with Merit from the Flickering Flames event, you can farm more of them from the correct location.
Best Candleberries location in ARC Raiders
The Candleberries item appears to be attached directly to the Cold Snap event. Hence, you’ll want to go on a map that has the event active. As of writing, this event is active on the Dam Battlegrounds map. Hence, I have marked the location on the same map.

If you’re on the Dam Battlegrounds map, head to the western part of the map near the South Swamp Outpost. Marked as a vegetation area, you’ll find Candleberries growing on shrubs in the open area. They appear to be pretty common in my experience, and you can harvest them just like you obtain other items, like Lemons and Olives.
I am unsure if Candleberries only grow on maps with the Cold Snap modifiers. However, you’re guaranteed to find them in areas marked as nature (on any map when the modifier is active). Another way to find Candleberries, a lot simpler, is from the Flickering Flames rewards track.

You will earn Merits for every 100XP that you gain from raids. Finding more Merits will allow you to hit different milestones present on the Flickering Flames. Some of these milestones will fetch you a decent amount of Candleberries that you’ll have to commit to complete the project.
If you need more, I have already marked the location on Dam Battlegrounds. I will look to add more spots for the berries once I have explored the other maps when Cold Snap is active on them.