For regular tires either:
A: Go to Les Schwab:fawkdancesmiley:
B: Cruise Craigslist, garage sales, etc...
Or
Get em from a cheap online source like this place
For larger tires:
Cut down big truck chains. You can sometimes find em on the side of the road.:haha: Also check craigslist.
For style of chains, the ones with the V-bars are very agressive, and if your going to be doing any pavement/snow driving, I wouldn't recommend them. But, they are good for bad ice conditions. They look like this:

Another thing to look at is to get the chains with Cam adjusters like these:

Another thing to think about, at least on wheelers is, you generally want to chain the front tires, as it will help ya steer and such.