Hi,
Anyone still looking for where Pages 5.5.3 stores user templates needs to navigate to the "Container" folder in the Library. From there, in this instance, you find the "com.apple.iWork.Pages" file and then right click to open a menu from which you choose "show package contents". Inside there is a folder named "Data." Open this, open "Library," open "Application Support," and there you will find "User Templates."
With any luck the user templates that have been saved in Pages 5.5.3 will be there.
I'm not sure why Yosemite or the latest version of iWork applications (whichever is responsible) makes it so hard to find files like these.
The obvious other thing you can do is what I did to find them - do a search of the top Library folder for the name of one of my templates. But of course you would need to know this. I suppose you could always save a user template in Pages with a distinctive name to search for.
Ordinarily of course most user template functions can be done in the chooser as has already been described. But I too needed to move old templates to their proper place when changing from Pages '09 to 5.5.3
I hope thjis helps those looking for these files. 🙂
Tim