How do you get the ambasador in tf2?

The Ambassador is obtained by achievement completion.

Milestone 1

Obtain 5 of the Spy achievements.

Reward: Ambassador

It can also be obtained via random drops through the game as you play. (But only after you have unlocked it as an achievement milestone)

It is also be crafted:

Class Token (Spy) + Slot Token (Secondary) + Scrap Metal

But you have a 50% chance of crafting the L'Etranger

DuhFog's Edit: also obtainable via the Mann co store by purchasing individually, or buying the spy starter bundle. (warning: not tradable for a few days and not craftable, therefore not tradable to TF2WareHouse)