The list below is compiled by user Zinkomania in Reddit that features the list of bosses that you can and can't inflict with poison. Credits to him and all the other helpful users who shared their knowledge to complete the list. The original post can be found here.
Gavlan sells poison-related items; be it offensive or recovery. Other than that. you'll find these items in very limited numbers as treasures and loots.
Boss | Note |
Ancient Dragon | Yes |
Belfry Gargoyles | Yes |
Covetous Demon | N/A |
Darklurker | N/A |
Demon of Song | Yes but it seems to recover automatically |
Duke's Dear Freja | No |
Executioner's Chariot | Yes |
Flexile Sentry - yes | Yes |
Giant Lord | Yes |
Guardian Dragon | Yes |
Looking Glass Knight | Yes |
Lost Sinner | No |
Mytha, the Baneful Queen | No |
Nashandra | No |
Old Dragonrider | Yes |
Old Dragonslayer | Yes |
Old Iron King | Yes |
Prowling Magus and Congregation | Yes |
Royal Rat Authority | Yes |
Royal Rat Vanguard | Yes |
Ruin Sentinels | Yes |
Scorpioness Najka | No |
Smelter Demon | Yes |
The Last Giant | Yes |
The Rotten | Yes |
The Pursuer | Yes |
The Skeleton Lords | N/A |
Throne Watcher | Yes |
Throne Defender | Yes |
Twin Dragonriders | Yes |
Velstadt, the Royal Aegis | Yes |
Vendrick | No |
Lost Sinner can be poisoned. She just has higher resistance to it.