Probably at any supermarket - shortening is simply a food grade fat. Crisco is the most common brand - in some countries you may find Kremelta. It's called shortening because it is used to make 'short' pastry - that is, a pastry with a high proportion of fat and very little liquid.
If a recipe calls for shortening you can substitute with the same weight of butter, margarine, lard or coconut fat. You can also substitute with the same weight of cooking oil but in that case you would need to reduce the volume of other liquid ingredients accordingly.
It depends on the recipe. Shortening becomes solid at room temperature while vegetable oil does not. So vegetable oil may be substituted for melted shortening only in recipes that do not depend on shortening becoming solid for texture when cooled.
Shortening
Not at all. Animal-based shortenings are all solid at room temperature, but vegetable shortenings can be either. Solid and liquid also behave differently depending upon the application and the working temperature. Generally speaking, solid shortenings are used to create 'flakes' inside doughs or batters.
1cup
Shortening
You "cut" solid shortening into dry ingredients, using the tongs of a fork or a utensil called a pastry blender.
Coconut oil which is a solid at room temperature.
Solid fats, shortening
Brand name for solid vegetable shortening.
I'll assume you meant butter for one of your shortenings. In most recipes, any solid shortening can be substituted for any other solid. The end product will vary some and in some cases it has to be shortening or it has to be butter. You will just have to try it both ways and see how it turns out.
Shortening melting is a physical change. It occurs when the heat causes the molecules in the shortening to become more energetic, changing from a solid to a liquid state, without altering the chemical composition of the shortening.
A solid fat made from vegetable oils, such as soybean and cottonseed oil. Although made from oil, shortening has been chemically transformed into a solid state through hydrogenation.