I scout Craigslist especially just before and after the major hunting seasons. Also, meat processing plants, butchers, fish markets, game processors, the grocery store, carnicerias and asian markets... plus my friends all know the situation and will call me up when they have a couple freezer-burnt chickens to get rid of, or whatever. That sounds like an insane lot of work, but it's really not. I don't check all those places weekly or even monthly. I know when the kill day is at my local meat processing place and will go out and bring back a haul of elk legs and green elk tripe maybe two or three times a year, for example. Or the local fish market for a 30lb box of sashimi-quality heads, tails, and chum scraps for $5 maybe every two or three months.
If it helps to think about it from another direction, consider that what you're paying for now is mostly indigestible corn. It moves right through your pets and you wind up cleaning it up from your yard or litter box. You'll end up saving money in the long run feeding more highly nutritious and digestible food because they simply won't need to eat the same volume of calories to be satiated and nourished (sound familiar? =) ).
It's hard to say with any degree of certainty what raw feeding costs, given different concerns about the cost and availability of animal foods, but most folks I know who are willing to do a little "hunting and gathering" are able to keep their costs very, very low. Others would rather spend the money for convenience and not the time. There's plenty of room for balance in whatever suits you and your pack. We save a great deal of money not just on the food bill, but vet bills too.
ETA: to answer the first question more specifically, I'll feed more or less anything that used to be alive. Somewhere around here I've got a photo of the inside of our pet food freezer which, at the time, had something like sixteen different species of animals represented =P. We'll feed marked-down grocery store beef one day, and wild-caught nutria the next. It also helps to remember that dogs are scavengers too and will eat just about anything they can get down their throats. My dogs don't seem to care how green or slimy a thing is, and they'll happily chow down on stuff that makes me gag to smell it. That's not to say I'm going around scraping up roadkill or anything, but certainly if my butcher's freezer goes down over a weekend I'm the first one he calls to come pick up a load of sour, slimy lamb ribs.
Also, if it's helpful, I wrote a Q&A for my dog club's newsletter a while back that answers a bunch of the basic questions: http://www.rawdogleather.com/DACARawArticle.html --I hope it's okay to link here, that is my commercial website but the article isn't selling anything ;0)