If you are in the US you can get normal Kennedy half dollars are any bank. The silver coins are not going to be cheap unless you get worn ones. What if really driving the price of those coins up is the price of silver and right now silver is in a boom. If you are outside the US? See if you can work a trade with someone in the US for coins from your country to the Kennedy halves.
Coin shops often have "junk silver," coins that are too worn for collectors, but great for us. They'll generally sell for the intrinsic value of the silver contained in them.
US coinage is widely available in Canadian coin shops, if that helps.
If you have a coin shop (or antique shop) near by ask them about their junk coins. But if you can't find those, try to check e-bay or invest from Jamie Schoolcraft. I know these aren't your cheap options, but if you're serious about coin magic these are a must investment.
Price of silver today is about what it was when I bought most of my coins. I believe I paid about $8-$10 for walking liberty half dollars, $11ish for Barber halves, and $25 a piece for Morgan dollars.
You generally want to get the lower grade coins that they have in the shop, nothing about Fine or Very Fine. Ask for the Junk Silver/Culls/Slicks.
If there isn't a coin shop near you, there's always ebay. Or alternatively (but somewhat more expensively) there is this.