Hey W. The ones you need are 3/8 Axel Nuts.
Home Depot and Lowes both sell them in the fastener section.
Neither seem to sell black plastic ones like Weber uses - but they usually have red, white, and metal ones.
Home depot's metal axel nuts are shaped like little top hats _|--|_ while lowes sells nickel plated ones like weber used in the 60s and early 70s (shaped like [-------])
Most restoration projects use the ones from lowes. They're 1.51 a piece.