Well, if you're strictly looking for a 68k powerbook, the 540/540c is a nice way to go. They're a little tougher to find.
I've got a 180 and a 180c and they're built like tanks and pretty cheap. I prefer the trackball to the trackpad on these older macs, so the 180 is my machine of choice. You can probably find one for under $25 on eBay.
I've also got a Duo 280, which is also a nice machine, but you need to deal with the whole external floppy thing.