Ok gents, not to power level myself here, but silver stacking is a huge hobby of mine and I can talk for hours about it.
Instead of rambling on and on, I going to give all of you the quick and dirty advice to make it caveman level simple for you.
If you are in North America, just buy ASE's (American Silver Eagles) or Canadian Silver Maple Leafs. These are government backed coins, but are also .999 and .9999 (respectively) pure silver. This means virtually no junk metals are in these coins, and normies will trust them because of the government backing.
Any coin store or pawnshop will buy/sell these coins. Just Google these coins to get an idea of what a reasonable premium is, as it goes up and down constantly. DO NOT BUY FROM EBAY.
EBay is full of scammers, and unless you have the correct chemicals and/or magnets, you can't test the purity of any potential metals you buy at some "too good to be true" price.
Reputable (but a little expensive) online dealers include Silvertowne, JMbullion, Apmex, and the Pinehurst Coin Exchange.
If you are in Europe, just go with the Austrian Philharmonic coin...cant speak to the quality (or lack of quality) of online dealers over there though.
DO NOT BUY "German Silver". So called German Silver is not silver that comes from Germany, but is an alloy that is made from copper, nickel, and zinc. Its typically sold to scam you.
Good luck, have fun stacking coins.