How cheap is cheap and do you want built up stuff or kits to build on your own?
I have found tons of good stuff on eBay for pennies on the dollar. But it really depends on what you are looking for. Alot of new kits on eBay will run you a few bucks, but compared to buying them straight from the manufacturer, or even online hobby stores, they are a good deal.
Yard sales are usually your best bet for built up stuff that maybe needs a little TLC. Just recently, someone a few hours away from me had an estate sale and a huge collection of train stuff in HO scale. The entire layout was up for grabs along with all the extra track, trains, and buildings that was piled in boxes. Four Benjamins and I could have had it all. Sadly I didnt have the time nor the money; that stuff was probably worth ten times that, but the owner just wanted it gone.
Keep looking around. You never know what might be hidden in all those ads and auctions
I bought an old Tyco kit containing three houses under construction. It included all the ladders and scaffolding. I plan to use them as fillers in areas of my layout that are under construction. Add a few trees and a couple pickups and it gives the illusion that there is work going on and progress will be made. As you acquire the buildings you want just replace the ones under construction.