You need a death flower, an angelfish, and a potion of youth.
If you have Realm of Magic, you can sometimes get the first two ingredients from the magic shop (the Potion Ingredients stall in Caster's Alley). If not, read on...
Death flower: You need to graft a pomegranate with an orchid. You can get pomegranates from the "gift" menu or fishing in Sylvan Glade. Orchids are a reward for space missions and completing a space collection.... sometimes. It's probably easier to graft lilies with snapdragons (in a greenhouse, because they're different seasons). If you're level 10 gardening and rich, you can spam-open rare seed packages and might get a death flower or an orchid.
Angelfish: You're just going to have to fish until you RNG one, I'm afraid (if you don't have RoM). I have good luck on the little peninsula behind the Willow Creek library and on the little island on Mua Pe'lam, but I've also caught them in Windenburg and Forgotten Hollow. I hear you can buy fish from the little fish stall in Ohanali Town during the fishing festival but haven't tried that, so I don't know if they have Angelfish or not.
Potion of Youth: this costs 1500 satisfaction in the rewards store.
Once you have all the ingredients, you need level 10 in both cooking and gourmet cooking for it to come up. Ghosts don't eat ambrosia lying around; they need to be in your household. That said, I had a child go to the fridge to get leftovers and eat my darned Ambrosia, so... your Sims will!
Kind of Cheaty: Some kind souls have uploaded both Ambrosia ingredients and full-on Ambrosia to the gallery.
