Hey, welcome. I am in the same boat as you. I have my own conversion going and my 24 year old daughter has been talking about doing a tiny house for years. When she saw my bus, she said she wanted one to do a tiny house out of. I found her a bus on Public Surplus Auctions. It was in Idaho, a 1991 Ward GMC 66 passenger (40 foot) with a 366 gas engine. She paid $825 total, tax and everything for it and we drove it home to the Tacoma Wa. area, 300+ miles.
We just got it last weekend and my wife and I are headed to Philly this week, so we have not started it yet or even have pictures to post.
So, figure out what you want, front engine, rear engine, conventional, flat front, gas, diesel, length, etc. Set a budget (have cash) and start looking around at Public Surplus, Government Surplus Auctions at Government Liquidation
, ebay, Craigslist, etc.
Great deals can be had on the public surplus web site, but can also see prices go crazy high.
For a Tiny House, if she is not going to drive it very far each year, you might look at gas buses. They go a lot cheaper that diesel and for a tiny house that is going to be parked a lot, they can be great deals. Like I said, we got a real nice 1991 GMC for $825, if it had been a diesel it would have gone for 3 time that at least. I paid $1500 for my 1985 Blue Bird/International with a diesel from a church and got a steal of a deal on it.
Anyway, let us know what you come up with or if you have any more questions.