If you own a NEC PC-98, be it a 9801 or 9821, you'll probably want a mouse.
Finding a mouse from Japan isn't hard, they're commonly on Yahoo Auctions, but sometimes they're either overpriced or unavailable.
A useful tip: Microsoft InPort mice are compatible with PC98s!
Here's an example of a quite common mouse to find in the US, the Microsoft C3KABUS1, which works great on PC98s.
I personally use this style of mouse on my PC-9821Ra266.
You do have to plug it in with the flat spot sideways, the connector is rotated 90 degrees compared to a NEC mouse, but who cares when you won't see it?
An additional tip for those of you with USB ports on your PC98s (like V166/200s):
USB-HID98 can let USB keyboards, mice, and joysticks work under PC98 DOS!
Update - 12/5/2023:
A real cool person who I met at CGC '23 that goes by backflip720 messaged me and said that some old Logitech MouseMan mice, specifically the Deluxe Bus (P/N 805155-00) model, work on PC98s! Big thanks to them for the tip!