While doing the "Following the Tread" quest I was able to loot a Necker gwent card off Hammond after I killed him.
Are there others who drop gwent cards as loot?
If you don't kill Hammond, can you play him for that card?

There are 3 Nekker cards. One you get from Lambert, one from the innkeeper in Harviken, and one random drop -- you got the random drop. Only the innkeep and blacksmith are interested in Gwent on Harviken, so I guess the answer is 'no'.

The one that the Prima guide (and all sites which copy it) says you get from Lambert, is the one you always get from Hammond.
Random drops come from playing, not from loot.
Lambert has Triss Merigold.

Yep. I was going off gwentcards.com, but they have since corrected their entry (on more than one card).