We also have a Honda Wave 125i, and we reshaped the seat yourself, but for somebody less adventures you best can go to one of the motorcycle seat/saddle makers/modifiers in Bangkok.
You can find a few on Sutthisarn Road, in Bangkok. Modifing a Honda Wave seat will probably cost you around 250 THB (also depends on the material you select).
Is there any way of altering the new Honda Wave 125i so that a big guy 6'4" (190 cm) can ride on it?
On a stock bike the drive position is too hunched and forward tilted so that shoulders and wrists start to hurt after a short ride.
Thanks for the info!