check for profession WQs in the map, these usualy reward 5expulsum
scraped trinkets always give expulsum, check WQ that reward trinkets
alch lets u transmute a few herbs into 5 expulsum everyday
u can also try to buy huge amounts of cloth and leather/bones, craft the cloth/LW bracers in an alt or just a new char (can be a lvl20) then send to ur engineer to scrap
plan in advance, if u can get rank3 recipes do it as they will reduce the expulsum alot
i remember at 1 point geting the pvp recipes was a super optimal low expulsum way to level engineer to max, check those too, as u can easily buy rank2 and rank3 with marks of honor
hope this helps