Window screens

At my house I have fixed window screens; they're all in one piece and can be removed, but if I have a single window and want to put in a window air conditioner, the whole screen has to come out. I have tried finding a replacement that are adjustable, but the only ones I'm finding are the ones that are for when the window is open slightly and you put in, but it's not permanently attached. Any idea where I can find these, or possibly buy a frame kit?