Great suggestion Mac!
I took a trip to 2 Ace hardware stores. Not all are created equal. In the future, I would go to a real plumbing supplier but it's worth a shot.
What you want is called a cone washer. You can see from the example that all they had were orange rubber. I would never put this on a bike but they cost all of $1.50 for the set so I thought I'd try it out. Makes for a nice contrast example I guess or cool on a custom bike. Cone washers have a straight bevel like the originals. "Packing" washers have more of a curve. If you want to make it just like the original with the steel spacer (I would), also look for one that is for 1/2" OD tubing. That fits perfectly on the steel spacer.
As for the steel tubing, they have rolled steel spacers for $0.60 each. You might have to cut it to the proper 3/4" length. You can also buy a long chunk of 1/2" OD steel tubing and cut multiples.
Turned out nearly perfect. The originals have a slightly rounded outer corner when molded but if you could just find some black ones, they would be nearly impossible to tell the difference.
In total a complete set with steel spacers would cost you about $9 at Ace, but you could probably get that lower. Just have to find BLACK cone washers.