Welcome, its good that you are asking before buying, should help you get the right bus for you when the time comes.
Auctions, Public Surplus: Government Surplus Auctions
Public surplus, https://www.govdeals.com/
Government Deals, and Ebay are the main auctions, there may be others too.
I got one bus at an auction and one off Craigslist. Check your local Buy & Sell, Craigslist, offerup, etc.
Check with your local school district and see when and where they sell their buses.
If you live in an area that cars rust, think about looking for a bus out West or in the South where rust is not an issue. The cost to fly and drive back will be less than the cost of major rust repair.
As far as the build goes, you can go as fancy and high tech or crude and low tech as you want, cost will follow. Nice thing is, if you lack skills, build it simple at first, learn along the way. You can always redo the parts you don't like later on. If you ask around, you will find that these are rarely ever "Done", more of a working bus, but "I want to change XXXXX".
Take a look at this post: http://www.skoolie.net/forums/f6/new...tml#post170823
it gives a lot of good info on bus types and you may want to look it over before setting your mind on one type.
Most of all have fun, its your bus, make it what you want. There is really no "wrong way" to build a bus. Make it your own.