Really maybe for fixtures but building materials bought peaicemeal are not ideal At least give us a clue as to what you are building for sure . We all love a deal but I think we need more info. Please post any other info, pictures etc in a comment here do not start another post.

Google “architectural salvage” and check out the offerings in your area. Ours is constantly posting new inventory — lighting, doors, slabs, cabinets, etc. Even some furnishings and decor items.

I don't do much online shopping. But I often hit the sales sections at bigger box stores and wait for sales. I hit the ReStores and other materials resale shops. Bigger cities often have places that have neat salvage finds. And more showrooms and materials dealers in general. Folks sometimes luck out at things like barn sales, shop closings, flea markets, resale and thrift stores. Hunt backroads for reclaims. A lot of people seem to find luck using local trade sites- but those can also have scammers or sketchy going on too. Craigslist, FB Marketplace, neighbor apps, those sorts of trade sites. Some areas have odd resale places, usually surplus or overstocks, that can be hit or miss on finds. The stuff is new, but often not the usual. Redistribution pallets can contain finds, but not for the faint of heart. Some folks develop relationships at the dead ends- places like junkyards. Or a contractor that has to pull stuff and it gets tossed if no one else wants it. And when good stuff comes in, you get to know :)

All depends on what one is looking for. Getting a pallet of lumber or brick, or wanting the consistancy of the same kind of paint used throughout a house, that would be more of a go the the store and get it on sale deals. Getting a cute light fixture for a powder room could come from anywhere.

Build.com is a good resource for decorative building materials like hardware, faucets & plumbing fixtures, lighting, appliances, etc. If you want really basic tile like white subway tile, you can buy 3 x 6 Daltile at Home Depot for ~$4/sf. After that, check out Habitat for Humanity's Re-store or any local building salvage dealer.

As for flooring, commercial carpeting is inexpensive as is porcelain tile.

And you can't really go to wrong with IKEA cabinets. Use their planners and installers.

I've purchased everything from build/ferguson so far... really racking of the frequent flyer miles. Haven't had much luck with habitat for humanity. I'm not looking for inexpensive materials... I'm looking for expensive materials for cheap. I did just get a spectacular deal on new Graff fixtures on eBay recently and realized I should be checking around more regularly for items I know I need to buy. I've also sold unneeded building materials quickly on offerup. I just thought there might be another particular source or app I'm not checking. Craigslist is a wasteland these days.

I bought 95% of our lights and all bath/sink fixtures through Amazon warehouse. I saved a chunk of change that way. This was 5 years ago.

I'm still a huge Craigslist fanatic. I got a 1930's vintage console sink with glass legs for $100 that I saw on a fancy salvage site for $3000. I've purchased solid wood doors, solid brass light fixtures for a steal. Sometimes the free section is a good place for building materials that people just don't have room to store and are perfectly fine. I've never had luck at Habitat. Have you tried Free Cycle and FB Marketplace?

Also, do you know how to use the google shopping feature? Type in what you are looking for along with used and ebay, craigslist, etsy and other sources will pop up. Much faster way to sort and look than plowing through individual sites.