Look online to see if you have a habitat for humanity outlet in your community. We have a couple here in Denver area that get quite a lot of stuff. Even new items (big builders are who I assume donate batches of extra fixtures, toilets, grout, tile, paint, etc.)
Check around, you'll probably find something close by.
You will likely have to buy a lot of your stuff from traditional suppliers, but you might get lucky and get a few bargains on some of the big ticket items. Also you can probably get lots of the $2-$20 items for under $5