You actually might try Aloris.
I needed some of those tabbed washers maybe a year or so ago because I was reconditioning a bunch of Aloris tool holders. They use those tabbed washers on their height-adjust knob so that things don't unscrew while in use.
All I needed was a couple of the tabbed washers to finish things off right, but I couldn't find the damned things anywhere, just like what you're finding.
Aloris doesn't have them listed individually on their site, but they do sell the whole height-adjust assembly as a replacement part, and they're not too expensive. Something like $12 or so for the smaller sizes if memory serves.
You might give them a ring to see if they have one. At 5/8" diameter, that might be a bit larger than they have in stock, but maybe some of their huge EA-sized holders use that size?
Here's a link to the EA-size. I'd just give them a call to see what they say.