Can you explain what you mean by a "pocket window"? It seems that the term is used several ways on the internet.

Are you looking to use this window as a way to pass food/drinks/etc from the kitchen out onto a deck/patio? This seems have become popular in the last couple years. By pocket window I assume you are looking for a window that slides into a pocket in the exterior wall just like a interior pocket door. The likely reason you are finding few examples/suppliers of this type of window is the inherent risk in this idea. A true pocket window would have a high risk of leakage (both air and water) and would be a absolute nightmare to replace when (not if) it fails.

If you are looking for a window to pass food/drinks/etc out from you would be better served finding a window manufacturer that you like and choosing one of the configurations they already have. That way you get a manufacturer's warranty. Say a glider window or a french casement. That's my 2 cents.

Pictures of your kitchen?