I have used
APMEX. My purchases have been a mix of one ounce bullion, ingots and junk silver.
One purchase I have made was
war nickels. These were produced during WWII because of shortages of nickel for war production and were produced with 35% silver. They are easy to identify with the large mint mark above Monticello on the back.
The reason for this was so that after the war the mint could pull them quickly from circulation. I believe because of the gold seizure many Americans bitterly had the same idea.
I have read on many forums how many like the junk silver dimes, but at the same time there is an admission that the coins have not held up well in circulation and have significant wear reducing the weight and silver content.
The nickels have held up much better and up to recently 15¢ (3 nickels) could still purchase a gallon of gas (based on silver content). The same price as when the nickels were minted.
Just something to consider if we go back to a bullion exchange system supplementing fiat currency. JMHO and YMMV.
P.S. For a long while they became very popular and were unavailable. They are finally back in stock.