Poison Wyverns are not found on the regular map on Fjordur, you have to travel to the Vanaheim realm. To travel to the Vanaheim Realm and get Poison Wyvern eggs you will need to use the Portal Cave (40.6 57.5),or hold down the R key to get an options wheel. Once there you can travel to the coordinates below to find Poison Wyvern nests.
Poison Wyvern Cave Coordinates
Once you find the cave it is time to get some poison wyvern eggs. I would recommend bringing a gas mask to protect against the poison. Poison wyverns are not incredibly strong, and it is possible to just fight your way through with the proper tames. I used a dimorphodon army to effectively remove all the wyverns from the cave on Fjordur. I probably wouldn't try the same formula on a trench with mixed varieties of wyverns in Ark Survival Evolved.
Another good strategy for stealing Poison Wyvern eggs is to use a Desmodus at night. Go into invisible mode and travel to all the different nests, select the best one, grab an egg and run. Even if you are still invisible the wyverns will chase you down, and they can easily get out of the cave. If you are playing single player you could mount some turrets at the end of the cave to deal with the wyverns or you could put some pillars in there. Another method is to bring a Fjordhawk grab and egg and simply die. The hawk will bring all your gear and the egg back to where you respawn.
Good luck grabbing a Poison Wyvern egg and hatching wyvern eggs on Fjordur!
Other Wyvern Locations on Fjordur