That is a good question. We know that Cartel just pedals paper and if they really have to deliver they steal it from the SLV ETF. I guess if you are in the business you have to do a deal with a silver mine producer to buy directly.
On a related subject, I took some visitors to the Canadian mint in Ottawa last week. They produce investment coins and collectible coins in gold, silver, platinum and palladium. I tried to buy an investment 1 oz silver maple leaf for $39 (it apparently was double struck and came in a plastic case) but there were none available, hadn't had any for a while and the sales clerk didn't know when they would have some available for sale. The actual mint operation looked like they were producing large quantities of investment grade coins on the manufacturing floor but they weren't going to their retail operation. Maybe all the future supply was spoken for by ?. I was able to buy tubes of 25 silver maple leafs from banks and coin dealers last year but haven't tried recently as I keep buying ECU stock instead.