Where to Find It

Kurzicks can find amber from several sources:
Monsters in Echovald Forest have a small chance of dropping amber when killed.
Kurzick NPCs in Luxon areas occasionally drop amber when killed.
Some amber weapons can be salvaged for amber.
The Kurzick Bureaucrat in House zu Heltzer sells amber in exchange for Kurzick faction.

Luxons can find jadeite from several sources:
Monsters in the Jade Sea have a small chance of dropping it when killed.
Jade Brotherhood NPCs occasionally drop jadeite when killed.
Some jade weapons can be salvaged for jadeite.
The Luxon Scavenger in Cavalon sells jadeite in exchange for Luxon faction.

In some cities, Rare Material Traders can sell these resources. Each faction's rare material is available in that faction's capital (House zu Heltzer or Cavalon). Some cities contested in Alliance Battles may have one resource or the other available for the sale, depending on who controls it.

Since players can collect these resources, they can also trade them: Kurzicks in House zu Heltzer, and Luxons in Cavalon. http://www.guildwars.com/gameplay/combat/amberandjade.php