The trainer for gym passes is in Payon, pretty much in the center, you give it to him and you receive +200 weight each use, max 10, meaning it requires 10 tickets, meaning you can get up to +2000 weight on a single character. And it's a quest skill or something like that that apparently doesn't affect the enlarge weight skill from merchants, meaning merchants could also profit out of this if they want to.
Just about 30 cells above the kafra. Should be a guy in a black suit and sunglasses. MIB.
The effect of that trainer is a little over the top but still fun to watch.
And searching for his location on a picture is just stumbled upon evidence supporting the kafra shop.
http://irowiki.org/wiki/Kafra_Shop