I just killed scourge boss and still haven't had a single one drop...

Thes dont drop from the bosses you find them in the open world ther are five per run the fith is a secret yet to be found

I found one at a corpse in one of the dungeons on Westcourt.

i have plenty of thees but im not sure how to use them, macabe dont have an option to use them

Ace sold me 2 of these at once for 3,000 scraps each a while after I started my 2nd playthrough on hard difficulty. She never offered more after that, not sure if something triggers it or if it's just luck.

Im on my 1st playthrough and a bought 2 from the vendor for each 3k. Now I have 8 Hearts so 5 per playthroug seems wrong? I grind very often for very long, and I heard if you spend more time grinding and dont go to the main screen or quit, the more you can buy from the vendor. After 2 hours I can buy 25 of the best Iron for my current state. And 2 times there was 1 Simulacrum for 3k to buy.

These always spawn on the same spot kinda . On Earth it will always spawn in the side dungeon that leads behind the locked gate on the 2nd overworld area(the one with the root with guns) , the side dungeon and the location of the item within are randomized but it will always be there. On Rhom it will always spawn in the Dungeon of the final area boss Claviger/Harrow (Though be warned if you have the Claviger in your world it can bug out and not spawn at all , had it happen twice and checked all other dungeons and open areas in the map to confirm both times) On Corsus it will always spawn in the first overworld area ( basically the first area you load into) On Yaesha it will always spawn in the somewhere in the Scalding Glade (The overworld area that has a night sky) These can also very rarely be dropped from Mini bosses (gotten twice) or found on NPCs such as the chick that sells ores in ward 13 ( found twice) or the crazy merchant with the root mask(found once). The price at the NPCs are 3000 scrap and only 1 person can buy them (not shared in multiplayer)

Does it spawn in adventure mode?

I think this is really hard to find.. It’s almost impossible to upgrade everything to maximum level because of this item. Maybe it should be dropped by defeating the final boss.