Yeah so this is currently one of the most controversial topics in the community. The need for Enhancement Cores in infusion is trash and discourages armor build diversity. You should basically be constantly switching your armor and weapons out as you get higher power drops, but if you get good perk rolls on anything you should save it for when you get closer to max power. Once you're high enough to do like the raid or something, then start infusing your best stuff. You can get Enhancement Cores from Gunsmith bounties, from the Spider in exchange for legendary shards, and from dismantling masterwork weapons/armor. You can also get them from getting to max rank in Quickplay and Gambit. It's quite the grind.