The last time I purchased some I ordered the IBM M-UY101 mice that now look to have disappeared. That worried me because I love these mice. They are just simple and reliable and haven't broken after 5 years of regular use.
Searching around I'm starting to feel uneasy that three button mice may be disappearing. One company was selling a refurbished IBM M-UY101 for around $100. They can keep it.
Apparently the Belkin F8E813 and the HP DY651A can still be found around. The HP mice are available directly through their store. The Belkins are harder to find and in limited supply. I'm ordering one of each to see if they look good. If they do I'm going to buy up a bunch. Hopefully my stockpile will survive the post three button world until I am no longer part of it.
If you find that I've bought them all up and the Internet now has been completely depleted of these devices, don't email me. I have only five words for you.
The mice came. The HP DY651A is identical to the IBM M-UY10. The Belkin F8E813 is a non-optical mouse, which can be really handy on some surfaces. The Belkin F8E813 is a little narrower but it is basically just a mouse, nothing extra special or troublesome. Get them while they last.