There's a lot of interest around investing in precious metals (PMs) these days. I've talked about the pros/cons of PMs in the past--you can check out some of my thoughts by following the precious metals tag. While I am not a PM fanatic like some in the survival community, they do have their place, and even if you're not interested in socking some funds away in silver or gold, it doesn't hurt to know about 'em.
When looking at silver, there's a variety of ways that you can invest in them. 1 ounce American Silver Eagles, Canadian Maples and other similar coins are readily available at coin shops, but they usually carry a "premium" of a several dollars over spot prices, sometimes substantially more if it's an especially rare coin or in highly graded condition. For example, an informal eBay search showed Silver Eagles at around $38-$42, with shipping, while the spot value of silver was only $33.80.
This premium doesn't come from the intrinsic value of the silver--it's one ounce of silver--but from additional collectibility/numismatic value. If you're socking away silver for a hedge against a major collapse scenario, you're more concerned with the intrinsic value of the silver than a specific coin collecting-specific premium.
That's where so called "junk silver" comes in. These coins are not really junk--these coins are 90% silver--but they're junk in the sense that they have little to no value above the silver they contain. What does that mean for the silver buyer? Well, it means you can buy "junk" silver coins at or near spot prices, avoiding paying a premium for a coin collecting value that is not important to you.
To get around spot prices, one could also buy old silver dinner ware, jewelry and so on, but the content of these will be suspect and they'll be harder to move than old silver coins. In my opinion, one ounce rounds like the silver Eagle and Maple will be easiest to move--they say "one ounce silver" on 'em!--and you may even get the numismatic premium back on 'em too. But, comparatively, old silver coins offer a good balance between near-spot prices and liquidity.
What are junk silver coins? Well, before 1965, most coins in the U.S. were composed of 90% silver. Nickels are the exception, while nickels from '42 to '45 contained 35% silver, the rest did not. So that leaves you with dimes, quarters, half dollars and dollars, in several different designs. I really like some of the old coin designs--the Mercury dime and Walking Liberty half dollar are two of my favorites. The Canadians held onto 90% silver coinage for a bit longer--they discontinued most of their silver coins in '67.
Buying silver is pretty easy. If you're got a local coin shop, I'd drop by and check them out--we have a pretty decent little shop about 15 minutes away that gives prices that are generally better than anything I can find online. Online, eBay has a pretty huge selection of silver coins in varying quantities. If you're looking to buy in bulk, Apmex sells 90% silver in bags of up to $1,000 in face value.
You'll often see silver coins sold by the face value; a rule of thumb is that every dollar worth of 90% silver coins contains about .715 ounces of silver. So, a $1,000 face value bag would contain 715 ounces of silver--yes, that's a lot of silver.
How much silver should you have on hand? It really massively depends on your individual circumstances, what you're preparing for and so on. It may not make sense for you at all--you'll need to figure it out. I personally generally tend to prefer gold to silver for collapse-type purposes, mostly due to its portability--one ounce of gold contains a lot of wealth; a few coins in the pocket can do a lot of problem solving almost anywhere there's a quasi-functional market.
One thing to think about--an emergency fund is one of your most critical preps, and it can make a lot of sense to include some precious metals in the mix. Our family fund is currently about 25% precious metals and 75% cash.