That's where you are wrong. There are so many opportunities to get cores. Scrapper bounties, spider bounties(which I dont suggest), turning in vanguard and crucible tokens and breaking down the rewards, turning in weapon parts to gunsmith, turning in materials to vendors to breakdown will sometimes reward cores, during bonus valor week in crucible you get 10 cores for resetting , breaking down drops will sometimes reward finest matterweeve, buying masterwork cores from spider(three a day). This is why I say cores are so easy to get if you just play the game.