Astrid assured me that one bite of Jarrin root would kill me. Well, she was wrong. Might have killed me if I only had 200-300 health, but not now.

Any more Jarrin Root around? Would it make a more potent poison than say Deathbell or Nightshade? Or is it all the same once its turned into a potion?

Jarrin Root is much more potent than a Deathbell or Nightshade. A poison mixed from it with the right ingredients and high Alchemy and good perks can allow you to make a potion with over 2k points of poison damage.

No other root in the regular game. You can find another one in the room with a sample of EVERY item in the game (only available using the console).
But then again, if you have access to the console you can give yourself as many as you want - of anything.