I recently read in the message board about a little house in Colossus Blvd, to the left of Mindy, where you can receive a free shield or use your training points for new shields. I never would have found this on my own, and I am now wondering if there are other houses or areas like this in all the worlds. If so, I would love to know where they are.
There's another hidden trainer on the roof of the Krokotopia pet shop, which is also hidden.
Watch the timer on the obelisks near the other shops, when they turn red all the way to the top the teleporter will activate and take you to it. If the timer's near the bottom try changing Realms, another one may be ready to go.