Thanks for that I'll check and see if they have anything that matches.
I'll post my findings.

 

I measured an old one I found in my parts box and this is what I came up with.
ID 7.93 mm
OD 13 mm

These measurements are for the bolt that goes in the bottom of the fork.
IIRC they are about a buck and a half from the dealer.

 

I measured one yesterday and got similar results.
OD: 0.521", 13.2 mm
ID: 0.312", 7.92 mm
Thickness: 0.035", 0.889 mm

Wasn't able to determine a HD part number for them though.
Not sure I would pay $1.50 each for them when I could basically purchase the HD Chrome Fork Slider Drain Screw Kit.
Next time I need to change the copper washers on a bike that already has a chrome screw, I'll investigate purchasing just the copper washer from a local source.

 

They aren't sold separately you get them with the bolt.
Part number 45405-75A