The short answer is:
Because our readers are often interested in footbags for a variety of purposes, however, we have split the longer answer up into three sub-sections below. Find the one that's most appropriate to you:
Footbags for kicking around: For people are are just looking to "hack the sack" and are not particularly focussed on doing difficult advanced stalls or playing the net game, just about anything labelled a "hacky sack" will do. Though, note that "hacky sack" is actually a trademark, many people refer to Guatemalan crocheted bags (and other bags you can find for under $5) as hacky sacks. You can find reasonable kicking bags just about anywhere -- at toy stores, kite shops, sporting goods stores, etc. If you're outside North America, it's a little harder to find them in stores, but if you look, you'll usually come across something usable. Contact the club(s) in your country if they exist -- usually someone else near you will have a better idea exactly where to get them from.
Footbags for advanced freestyle tricks: Depending on your level of play, or your desire to get to an advanced level, you may be more concerned to get a really good bag for freestyle (stalling), and therefore you may be willing to pay a little more. Good freestyle bags (usually made from the "facile" material, and made with 32 panels and filled with either plastic beads or sand) generally cost between USD $20 and $30. We strongly recommend mail-ordering good footbags, unless you happen to live in a town that carries professional freestyle bags (good luck!). Contact Flying Clipper or the World Footbag Association to find out the best way to get a sweet facile freestyle bag.
Footbags for net: There are very few net bag manufacturers out there. The best way to get a net bag is to go through the World Footbag Association's online catalog -- there's really almost no chance you can find one of these types of bags in a store. If you want to play net, you should do yourself a favor and get a bag designed for net. There are several manufacturers, and the WFA tends to stock all of their products.