You can buy rank 6 and 7 snacks with gold, but the higher "mega" snacks only come in packs, as drops, or from plants.
If you are getting serious about training pets, invest in Evil Magma Peas, Deadly Helephant Ears, Sword Ferns, or Couch Potatoes. Those crown plants are well worth the price for the return in snacks, reagents, and treasure cards (which can be sold for gold). If you don't want to spend crowns, there are good rank 8 snacks as drops from Mirror Lake, and occasional 9s from WaterWorks. You can also farm for Couch Potato seeds in Grizzleheim. Prickly Bear Cactus seeds can be bought with gold. They drop mostly lower rank snacks, but occasionally some 8s.
No. Rank 8-9 snacks are only available through mega-snack packs, gardening and certain battle drops. They cannot be sold in the Bazaar at all. If you are a serious pet trainer and want lots of high rank snacks, I'd suggest you take up gardening and grow couch potatoes.