I have a Shaman that can’t survive Nazjatar World Quests and completed BFA Pathfinder Pt.1 with just changing specs during that process without having his self resurrection on cooldown, and became exalted with Zandalari, and Vulpera, Honorbound, and if BFA crafted gear when scrapped, had the same amount of expulsom then I could refund some non-soulbound Blacksmithing items I scrapped and send them to him and then he could start an Abyssal Focus and make a 415 Ilvl Engineering helm, which is 25 ilvls for him, and just 20 on my Blacksmithing pants made Nazjatar much easier, and Mechagon, so 8.2 could be more livable for him, it took 101 scrapped Blacksmithing items to reach 30 expulsom for my 20 ilvl upgrade on my pants, which also emptied my own guild bank, of BFA plate I leveled Blacksmithinig with so also if it changed to scrap items for the amount of expulsom as when crafted cause I probably spent somewhere between 4 to 6 expulsom on each item so if that changed, I could save some that plate in my personal guild bank, and send to my Shaman, to help with any 8.3 or other recipes, for expulsom, and once he completes an Abyssal Focus, he can upgrade his Engineering helm, with that additional raid quest for the upgraded Abyssal Focus, if he needs more ilvl than that to survive 8.2 content.
Your title and post are a little confusing. For crafted gear and world drops, you should have the same 1 in 6ish chance at scrapping an item for expulsom. You don’t have to craft the item yourself, you can just buy it off of the AH in bulk and hope for a proc. In that way, expulsom can be purchased, it isn’t really random.
If you craft the BoP items for your armor crafting profession, I believe the boots and pants for blacksmithing, you should be able to get all of the BoP reagents from the crafted item back in order to re-craft the item over and over until you get the stats you need on the gear. So once you get 30 expulsom or whatever for a pair of pants, you can simply craft-scrap-craft the item over and over until you roll the stats you want. As soon as you get an upgraded set of pants, you can scrap the crafted ones to craft the boots. You will need to supply the ores and fluxes still to craft the items.
However, the expulsom (which is also available from WQs), is BoP. As are the items that return massive amounts of expulsom. So you can’t craft them for an alt or trade them.
If you are extremely undergeared for Natjztar, hit the AH and look for cheap BoE gear. I found rings, weapons, etc… for extremely cheap on the AH…2k gold each maybe. Focus on one character to unlock the zone, during which time you should get a bunch of mana pearls. Benthic gear can then be unlocked for most slots for 5 mana pearls each, resulting in 385 items. Also, consider alternative bodyguards in the zone. Neri is a dps character who will eventually stun mobs. But the other two options are a tank (assuming you single-pull mobs) and a healer (assuming you are good at kiting / staying alive long enough for the heal).
Hope that helps.