Easily found at any local home improvement store- it comes in many varieites, sizes and price ranges.
I suggest working on something more heavily weighted than canvas--like a piece of wood.
Any old piece will do. What I tend to do is go to the thrift store and look for a picture that is done on wood and cover over it.
All you need once you have your joint compount is something to spread it with--plastic knives work good if you don't have a palette knife and some tools to make your impressions or to carve with. You don't need any thing fancy, just stuff you find around the house
Some suggestions: wood skewers, knitting needles, plastic bowls, lids from small bottles, pieces of screen, stencils...well I'm sure you get the idea...anything that is textured that will leave an impression works.
Spread your joint compound out on your substrate, let is sit about 5 minutes then begin to play- draw, doodle, carve, impress with your stencils or tools to create your design.
NOW LET'S LOOK AT SOME EXAMPLES:
Here is a piece that is started...
The texture on the left side was done just with a plastic knife which leaves nice ridges.
The top portion was first impressed with a small piece of one of those green scouring pads, then I pressed in the circles: large one was a plastic yogert container, the mid size one was the mouth of a small plastic jar, the small one was a top from a supplement bottle.
The next section the texture was created by scribbling circles with a wood skewer.
The section following that used the same skewer to draw straight lines.
The bottom right side has a small torn section of textured wall paper that had tiny bumps on it. The compound was spread across the bottom section- then impressed deeply with the green scouring pad, another circle was pressed in, and a few lines were drawn with the skewer.
It is set aside to dry, which can take anywhere from 24-36 hours to dry completely since it has some thickness to it. Once it is dry, I will cover the entire piece with a mix of 50/50 white glue and water- it helps to hold the dried compound on the board without any flaking off.
A picture was drawn on paper, then glued to the wood. Right now the face is lightly painted in. The joint compound was spread arond the face to create hair. The wood skewer was used again to create the hair texture.
This one is also drying. I will add the glue/water mixture once dried and then begin to finish the painting.