You can order plexi online. 1/4", smoked plexi is common. Leave the mask on. Use the old window as a pattern, and cut the new window out with a saber saw. Sand the edges. Tape the window on the boat and draw a line from inside around the mask. Remove the mask around the edge outside the line. Sand it lightly. Any part that will have diberglass behind it but not be glued can be painted black to hide the fiberglass. Glue the windows to the boat using Dow 795. Tape them in place til the adhesive dries. Then remove the tape and the mask. Screws are not usually needed. In fact, unless you oversize the screw holes, the screws will crack your plexi as it expands and shrinks with temperature change. Check your adhesive yearly to make sure there are no holes or separations.
If you don't want to receive plexi by shipment, sign shops deal in plexi. They will order it and let you pick it up.