i would likes mega snacks too because i would be able to level up my pets faster like my saber tooth and my night hawk. to get the mega snack packs are expensive but you can farm places like waterworks or even high level bosses to get them to drop but it might take awhile.
The plants I know will always drop mega snacks at elder are Couch Potatoes, Evil Magma Peas, Fish on a Vine, and Sword Fern. I plant all of these and they always drop mega snacks. Other snacks that have a chance of dropping mega snacks at elder that I know are Maelstrom Snap Dragons, Red Bell Peppers, Bread Bushes, and Queen Crape Myrtle. Hope this helps.
I myself have a large plot of couch potatoes, and a medium-size plot of evil magma peas. Both of these reliably give mega snacks, and also reliably give seeds to re-seed themselves. The size is determined by what fits inside the area of a large area-of-effect garden spell, and a medium area-of-effect spell.
That is enough to keep me pretty well-stocked with mega snacks, as long as I am not too crazy in doing pet games. I don't want to spend too much time gardening, and I don't want to have to pay for energy refills either.
Be sure also to go to Wizard101 central and look up the "likes" for the plants, and have those dropped around your plots, so you get good yields.