Blinds to go is customized only. Homedepot Rona and sears home carrys lots of brand names so both customized and already-made.
Bouclair has already made as well. All of these store can do cut down size.
You d better bring the measurement. Length width and depth the room for brukets if you prefer install the blinds inside of the frame. Otherwise you have to consider where to mount it. Measurements best get from 3 spots for both length and width since lots of windows will be a little bit off.