You can get PCBs made in China quite cheap, theres a few sites that can import them. But you will have to design the PCB, and solder on all of the components (you can use soldering paste for SMD components).
Overall, if you're starting from no knowledge, its quite difficult, or maybe very difficult. Depends how fast you can learn. But once you've got the process down, you could make all sorts of PCB sizes.
The easier thing to do is get a teensy, 3360 sensor from tindie, and connect it up to the existing PCB inside the mouse, after trimming off anything unnecessary (like the old sensor).
I made a guide for the IE Classic, if you look in the description of the video below, theres a link to a PDF which explains it step by step:
If you're determined to make PCBs, its probably a good idea to use the same design as the teensy and tindie 3360 PCBs (you can find their schematics online), then you can just use the same firmware from the IE classic. Otherwise you'd have to write your own firmware as well.