1. . Open the Terminal App (in Utilities)
2. Type: find . -name "*.template" -print
This should find and print all the template files. Now, let's assume Pages has been using a different file extension.
1. Open Pages, go to your 'My Templates area in the Templates viewer.
2. Copy a filename of one of your templates. Let's assume one is called 'mytemplate'
3. Open the Terminal App (in Utilities)
4. Type: find . -name "mytemplate.*" -print
(Replace mytemplate with your own template name.
These should find your template files on your old Mac. Be careful with spaces in file names and directories... Failing that, sounds like a visit to a group meeting where you can get some help. :-)
Note, I have checked where Pages v5.8 places it's template files and they are in the same place as later versions... so where they are on yours is puzzling.