No shortage of serial trackballs in my local shops, although most of them have the "IBM" (round) connector that's commonly used for the mouse port on PCs rather than the short in-line "serial port" connector. A conversion plug should be under $2 US if that's the problem. Most conventional mouses came with the convertsion plug a few years back, before the PC mfrs all switched over; but I can't say whether any of the recent ones still have it in the package.

The only other "common" connection is to USB, but you have to look for, and pay extra for, a mouse or trackball with the USB hookup. There are "converters" to allow you to connect serial devices to USB ports, and they sometimes work okay. I haven't noticed adverts for USB device to serial port converters, but they may exist - "Universal Serial Bus" is just a sort of "fancy" plain-old-vanilla serial connection. A query to the knowledgeable person at your local supplier (the one who's never there when you go in) might get you some help.

John