When shopping for a bus, you've got 4 main options, which I'll list here from cheapest to most expensive (usually—there are always exceptions).
For your average un-converted bus, most people seem to spend between $2,000-4,000, although you can find ones in poor condition for less, and newer buses will of course be more.
1. Online auction
One of the cheapest ways to get a bus is straight out of service from a school. You can give them a call and find out where they auction them off if you'd like, but a good place to keep an eye out are the two main auction sites Public Surplus and Gov Deals. The downsides to these auctions is that they typically don't have a ton of info about the buses, and I've even seen a decent amount of inaccurate info. You often can't test drive it, and sometimes due to the short time-frame of some of the auctions, you just have to bid blind without checking out the bus first. And there's a time limit on when you have to pick it up, which can be difficult if it's across the country and you don't have a flexible schedule. Plus side is that they usually haven't been sitting too long, since they're usually fresh out of school service. And if you're lucky, you'll get the full maintenance records included with the sale!
Usually a bit more expensive, eBay tends to be people who bought the bus at auction and are just flipping it to make a bit of money (which might not be a bad deal, since some of them will know a lot more about the bus, and be able to give you more detail). Or it's people who are selling a bus in some stage of conversion... in which case if it HAPPENS to be along the lines of what you wanted to do anyway, you might find a good deal. Do be careful of scams here, and use eBay approved payment methods.
Same as eBay, usually a bit more expensive, but I've seen some good deals. Setup saved searches for the areas you're willing to travel to, and get alerts when someone posts something! Don't be afraid to bargain with them!
Although there are sites to allow you to search across multiple regions, one way that I've found to be effective and quick is to search an area, then change the city in the url to other cities, and it will keep the same search criteria. So for instance, if you're in Portland and the search terms are "Cummins" and "bus" and you want only posts with photos and between $1,000 and $4,000 you can setup that search for one city (I'll use Portland in this example) and then view the results. Now to swtich to a new city with the same search just look up in the URL bar where you'll see "http://portland.craigslist.org/search/sss?query=cummins+bus&sort=rel&hasPic=1&min_price=1000&max_price=4000"
Select the "portland" and change it to "denver" to do the exact same search in Denver. This is a really handy way to setup several searches quickly, then just save those searches to get notifications whenever a new item that meets those criteria is posted!
Usually the most expensive option, but I've seen some decent prices anyway! You'll get the benefit of being able to test drive it, and often some type of warranty. Search for dealers in your area, and if you've found a good one let me know and I'll add it to my list here!
Pacific North West
Things to remember
1. Don't go too cheap on your bus purchase. If your overall budget is $30,000, it's kinda silly to try to save $1,000 on your bus by buying one that needs a more work, or isn't exactly what you want. The difference between spending $30k and $31k is pretty small.
2. If your on a tight time schedule for getting your bus, or really want a particular bus (like an older Crown or Gillig) be willing to pay a bit more if you see a bus you really like. Don't worry if people say you paid to much, the value of the bus is whatever you're willing to pay for it. Of course, if you've got a few years to poke around and shop, then you can be picky.
3. Don't forget that you're buying an old bus, and there's an element of risk no matter what you do. I bought my bus from a guy who buys and sells buses, and was willing to pay extra to get the exact bus I wanted (Gillig Phantom) in the time frame I wanted (I had 2 months to find a bus). I immediately took it to a diesel mechanic who specialized in Detroit Diesels for a full inspection and service ($1,000k). Six months and 2,000 miles later the engine ECU died, requiring a $2,500 repair. That's just the name of the game, and the risk you take.