If you have eso plus and can stash it all in your craft bag, there are lots of ways to get them in dribs and drabs.
- alchemy writs. Your character needs to carry 8 potions and poisons. Each turn in costs you about 0.3 reagents, and you get rewarded six plus the occasional survey that rewards 40ish.
- Dark Brotherhood shadowy supplier passive. Her toxin satchel will have alchemy mats half the time, and useless potions the other half. When you get reagents, you get nine and they’re the more valuable poison focused ones...emetic, nirn, nightshade, etc.
- for straight up farming, dlc zones tend to be resource rich. So do the first zones, away from the crowded cities (Auridon, glenumbra, stonefalls).
- buy them. I can farm enough raw materials to refine and get $40k in gold tempers to sell way faster than I can find alchemy nodes. Watch the stores...I just bought 200 columbine for $8k, those sort of deals can help build up the supplies. I could even split that stack and sell 100 back for $8k if I needed the gold.
- alchemy writs. Your character needs to carry 8 potions and poisons. Each turn in costs you about 0.3 reagents, and you get rewarded six plus the occasional survey that rewards 40ish.
If you're on PC and have access to the portable banker, you can use the addon "Dolgubon's Lazy Writ Crafter" to avoid having to carry those potions and poisons on each character you do the writs with. It has an option to auto-pull writ items from the bank.