Check out the Consumables and Buffs section on Wowhead:
The guy who writes this guide is super active in the DK discord as well:
https://discord.gg/worms
Flask of Stoneblood is typically your best raid flask, but if you haven’t hit def cap, and have some stuff from tank loot boxes from tanking dungeons, you can could use a defense elixir. The current content is fairly faceroll, so you shouldn’t have any trouble with it in prebis.