Jetsam can only be obtained on October 16th (Jetsam Day). On that day, you just go to the Create a Pet page and the Jetsam should be selectable.
You can get a jetsam by creating one somewhere near the month of October because the jetsam is a special edition pet, also you can get a jetsam from the pound if someone abandons a jetsam (which is very uncommon), you can get a jetsam by giving one of your pets a some kind of jetsam morphing potion, or you can use a magical plush toy.

P.S the last two ways to get jetsam is very hard because you need to save up lots of neopoints. And by lots I mean LOTS!

Jetsam is a noun.

Jetsam derives from jettison.

There was a lot of flotsam and jetsam along the shoreline.