Two questions about gelatin
In most parts of the country Knox is the most common (or only) brand of powdered gelatin. Some cookbooks refer to sheet gelatin, but I've never seen in a store. And for kosher cooking I read of a gelatin derived from fish.
I've made some highly gelatinous stocks using parts like feet (pigs or cow), but I've never tried purifying it to the point that it could be used for dessert.
Knox powdered gelatin is the most commonly found supermarket brand, sheet or leaf gelatin is used in professional kitchens (but there's no stopping you from using it) and Amazon, King Arthur Flour and various restaurant supply places or possibly cake decoration/craft shops will have sheet. No difference in the end result between the powdered and sheet, IMO, except the $$ and ease of obtaining.
Agar, Carageen (or Carageenan) and Kosher gelatins, like Lieber’s, Carmel’s gel, KoJel’s gel, and Hain Superfruits are available as vegetarian alternatives. These gelatin subs are usually found in your local health food store or a larger well-stocked supermaket such as Whole Foods, etc.
If you go the agar route, here are some basic rules:
Substitute powdered agar-agar for gelatin using equal amounts.
1 Tbsp. of agar-agar flakes is equal to 1 tsp. of agar-agar powder.
Set 2 cups of liquid using 2 tsp. of agar-agar powder, 2 Tbsp. of agar-agar flakes, or one bar.
Highly acidic ingredients, such as lemons, strawberries, oranges, and other citrus fruits, may require more agar-agar than the recipe calls for. Also, enzymes in fresh mangoes, papaya, and pineapple break down the gelling ability of the agar-agar so that it will not set. This rule also applies to regular gelatin, which will not set when combined with fresh pineapple. I've never tried mango or papaya in anything gelled, seems wrong.
There are also vegan rice starch (A & B Ingredients brand) and soy-based gelatin (Nu-Soy Gel brand from SoyFoods USA) alternatives, but I know nothing about these products.
So, for your use and shopping convenience, Knox is probably the way to go. Find it in the baking aisle.