Does anyone know where all the anniversary letters are?

Aussie_Kangaroo

I could only find the one in Miss Claira's flower garden.

Holly

I found one at the Tropical Jungle in Western AGE. I'm still searching for them so if I find anymore, I will come back here and update my comment. 

Tropical Jungle 

Space Corridor 

Green Dragon Tavern 

Icetastic Shop

Tiki Tropics 

scooterbrat

Zoo Express

Zug Tug Store  (inside)

Ned Kelly's  (left of "s" on cabin)

Dragon Mt. Entrance

Ms. Claira's Flower Patch

Ship in Bottle