I have an Elecom Deft, there's also the Deft Pro which I may upgrade too. It's essentially the closest thing to a MTE. Ebay and Amazon have them.

What xkfm said: Amazon, and Elecom or Sanwa Supply. I had a Sanwa Supply and it broke after a couple of years (switches died). Not really a big deal but I'm now using an Elecom and it's been fine for the year or so I've had it. I wanted thumb operated and wired, for which there is no other option AFAIK.

I use an Elecom HT1DR and like it quite a bit. It is their "huge" trackball and I have found it to be pretty comfortable in daily use. The only issue I have is that some of the extra buttons don't work on Linux with the current driver.