Look for #4 flat washers. Then in the product details look at the diameter measurement, and see if that matches what you have. If not, go up or down a size (ie, #3 or #5) until you find a match.

Those are pretty weird. What I would do is find some that are the same width and drill them out to the inside width.

No, those size washers are readily available. I had to find them a while back. I just don't recall what the exact size was at this point.

My bad.

I recall reading about those in a thread somewhere here on pinside, think it was in the white water club. The guy couldn't find them anywhere to buy as parts and ended up buying some long, chromed metal tubing that he cut/sawed off into correct length and used with good results.

I think the OP was looking for the washer that go under the tubes, rather than the tubes themselves. Oh, also, if it's not a #2, or #4, it could be a 3/16.

I also found the tube posts a while back. I'll have to dig through my past orders to find the vendor and part number.

[edit]: It's a spacer. Here it is: https://www.boltdepot.com/Product-Details.aspx?product=25944

So, since that's a #4 spacer, the washer the OP is looking for is also likely a #4 flat washer.

https://www.boltdepot.com/Product-Details.aspx?product=4282