Silver comes in many forms; crystal, ingot or bars and coins or rounds and buying it for investment or keeping.
Many collectors prefer "first strike" or proof like coins, while investors start with small ingots and rounds depending on their financial status.
Buying Silver as a collector is best done through specific Ebay Shops and Dealers.
The small Investor can buy direct from Ebay sellers but make sure that their feedback is specific and they know what they are doing. Look for sellers that have different types of silver on offer and email them to discuss your needs, they may be able to help you source other types.
Large purchases may be made through the Mints themselves - BUT only if they export to the UK. Most will not.