If you are only going to use it a couple times then rent. Check out the boat
shops. Personally, I use inflatable jackets from Stormy Seas. They work well
as regular jackets, have big pockets for survival gear and other
necessaries, and will keep you afloat in a pinch. It would be perfectly
reasonable to make one of these things your regular flying jacket. Pretty
cool. They make versions of everything from vests to heavy parkas.
http://www.stormyseas.com/home.html