Maria, I believe that you can find gum arabic in art supply stores. Look for stores that specialize in artists paints and probably offer framing also. Craft supply stores probably will not carry it.
I believe that you can get the same effect with hide glue but I can't remember if you have to use water based paint for this or not. Hide glue is available in woodworking supply stores and probably some home centers. I can do a little research if you are interested in this.
One technique that I use for "peeling" paint is to glob rubber cement around in random patches. Do this over a good finish in a different color than you want for the final color. Then you paint the piece as you normally would with the final color of paint. When the paint has dried, just take your fingers and rub off the top layer of paint and rubber cement. This leaves spots of the "old" color paint showing through. I don't believe I would use this technique on the same piece with the crackle finish though. You can overdo the "aging".