Here is my recent experience. I am ashamed to say that a few days ago I sheared off two of the rivet caps (covers) when backing into my "Scamport". They were those holding one of the curtain rod brackets, size 56 on the Scamp parts store page. They describe them incorrectly as 9/64 Dia., they are actually 5/32 Dia., 9/16 long. All others on their page are 1/8 or 3/16 of various lengths. I had to go to Home Depot to find that size, the smaller, local hardware stores only carry 1/8 or 3/16. That 5/32 is an oddball. Aren't fractions fun?
Aluminum was easy to drill out after grabbing the inside end with pliers. To avoid accidental damage, I did that by hand. Sealed the replacements with a dab of Lexel adhesive/sealer. Note: it is worth spending some money on a decent (medium price) pop-rivet tool. I keep one in the Scamp, along with a small bag of spare pop-rivets.
Here is a link to a nice page showing various sizes:
Standard Open End Rivets : Hanson Rivet