The Gladiator in Mount Olympus is pretty good. Ra also drops. I believe that the other secret bosses do so too. Please consult this page for a complete listing. (Click here)
Honestly, gardening for amber is the best way without burnout in farming dungeons for it. Check out wizard101 central wiki for plants that may produce amber.
Amber gathering is a slow slow process, but it can be accomplished. I have crafted all the spells, and many other items that require amber. I still have 300+ left, and it continues to grow.
I don't farm dungeons. I have 2 37-plot King Parsley that gets me what I need.
