They Are Widely Available
Here in Albuquerque, they are available at Central Trailer Supply, a retailer who specializes in generic RV parts, appliances, holding tanks, and hitches.
I'm sure there are many such RV parts houses in Illinois.
(Do not use a flat washer from Lowes or Home Depot. They are made from soft steel that will flatten, distort, and fail to keep the ball in adjustment)
2007 Classic Limited 30 (Sold)
2007Chevy Silverado 2500 HD Duramax/Allison
Four Corners Unit WBCCI #8654, Affiliate NM Unit