Silver bars and ingots, the very large commercial bars, come in a variety of sizes and weights. Usually all of them have a fineness of 999 pure. This means there are 999 parts per thousand of pure silver. One of the disadvantages of silver bars is that some countries charge a tax for purchases whereas they do not for coins. Perhaps because coins are usually legal tender and bars are generally purchased for investment purposes.
All silver bars should carry the weight, the fineness and the mint at which it was struck. Sometimes other information such as the assayer is stamped on the bar also.
Silver bars are available from 1 troy ounce up to 1 kilo and ingots can be up to 400 troy ounces. Ingots tend to be less polished and are basically rough bars of solid silver. They are used by jewelers and commercial companies using silver for industrial purposes.
Silver coins of course are very popular. Many hundreds are available and some collectors like to specialize in a particular coin such as the American Silver Eagle or the Canadian Silver Maple Leaf, for example. Most major countries and some smaller ones produce silver coins including:
AustraliaTo name but a few.
Italy (also Roman)
A good browse on google.com will net you heaps of sites that buy and sell silver bullion.
Always ensure, when the time comes to make a purchase, that you pick a reputable dealer and ensue that with each purchase you get a certificate of authenticity for your silver bullion.