What's an easy way to get them? And the ones that say "Holiday Event Item" do those return every year or are they only released once and never return?

By far the easiest way is to buy them from other players using the broker or forums. Other than there are some common items that can be obtained through fighting mobs. Festivals usually have common avatar drops too. Leveling your team will make it much much easier to get avatar items when fest time comes around.