This is for the US, not sure about elsewhere, but…

The easy and reliable way for most people is probably just bikesdirect.com
I’ll fully admit that their website looks like you’re immediately gonna get scammed, but it’s legit and I’ve ordered from them. Don’t just trust me though, search around and find people who have posted and can verify that they’ve ordered and it’s legit.
They’re importers who sell branded and off/non branded bikes direct from the manufacturers at a heavy discount. Shipping is free, but it takes a pretty long time since they’re shipping it from China

The hard and less reliable way is to ask around. I got to be good friends with the owner of my local bike shop, and he’s the type of person who organizes races and does a lot of custom work, so he knows a couple people who run small bike factories/workshops in the US. They sometimes have bikes that have some marks on the down tube or a slightly messy weld etc. but otherwise perfectly great bikes that they can’t sell. So it’s definitely a long shot and more likely in cities with a big biking scene, but this is where you get the best deal. I got a frame for $300 and they would have sold it for over $1500 but didn’t because the holes for a water bottle cage were milled slightly off center.