Power 'brick' or a power bank? If you want the AC connector anything that supports QuickCharge 2.0 or at least 2A output should be fine (the latter won't do Fast Charging, though). I use older Samsung bricks and a couple LG (from the V20 and G4). Anker is a good, popular brand for chargers.
For Power Banks, same thing goes. I'm using an Aukey one, and while it works, it doesn't support QuickCharge so it's just standard charging at 2A. Prices have gone down significantly enough so that QC2 (or 3, but the S8 doesn't support QC3, so that's just 'extra') are pretty cheap now.