I try to find larger swamps, preferably with a nearby meadow to make a safer outpost (with a portal) at. Though even the bigger ones can be weirdly empty sometimes. The one currently near my base apparently has abomination spawns all over the place (yay root armor at least) but only had one crypt, a ruin i couldnt even access because it spawned mostly underwater (i could see the chest but even the jump "dive" wouldnt be enough to reach it, so frustrating), and not even a geyser to make a core farm at. Just a whole lot of witch huts mostly. If the crypts you did find show you where your nearest bonemass is, the swamp bonemass altars spawn in are usually great for finding crypts as well.

If you really can't find any crypts then yeah, your only real alternative is muddy scrap piles on the surface. Just like silver you can sometimes get super lucky and find a pile that's just sticking up out of the ground but most of the time it's more hidden underneath. Wishbone does reveal these but if you don't have that yet, you have to use something like stagbreaker to just randomly smack the ground basically and look out for when it tells you "Too Hard" for no apparent reason. You may have hit a scrap pile under the ground with stagbreakers AoE attack. That, or spend a lifetime hunting oozers. Those aren't exactly the most common enemies and they aren't guaranteed to drop any either, just a chance of it.