Get a used mouseman from ebay ($10 and free shipping), throw away the top cover, and 3-d print your own.
Don't own a 3-d printer? Probably one of your friends does, or the local university, or the local hackerspace, or as a last resort you can use shapeways.
Grab some modeling clay in your hand, make a 3-d scan of the resulting "handle", add fasteners for the buttons and ball (or IR chip), then 3-d print a custom-grip top cover. You can get IR mouse elements and ball elements from old mice, usually for free on Craigslist. Or the local Salvation Army store.
Purchase a sheet of friendly plastic (polycaprolactone), soften it in a pan of boiling water, then lay it over your relaxed open hand like a handkerchief. Wait for it to cool and harden, take a dremel to it, and use that as a custom-molded mouse top.
Get an Arduino, or any of the zillions of hobbyist microcontroller systems (pic, propeller, &c) which have a USB interface, and add buttons and an IR chip from an existing older mouse and program the buttons specifically for your needs.
Get a used mouse with lots of buttons, remount the buttons into a custom top as mentioned, then reprogram the button codes in the driver.
Or write your own USB driver at the OS level - it's not that hard. (For windows, it involves downloading the DDK and modifying an example found on the net.)