You can use the heaviest Pellon interfacing, too, but it isn't as stiff as the Timtex. I got it at JoAnn's, because they and the local LQS didn't have the Timtex. I used it for some smaller / shallower bowls and it did OK.
I wasn't real happy with the first fabric bowls I made, because the satin stitching left little straggly threads showing. I sat in on a demonstration and got one tip that wasn't in the book. Sew your satin stitching at least three times. Do the first seam on the top side, flip the bowl and satin stitch the back side, then flip it and stitch the top side again. This will give you a nice, smooth join with no little straggly threads.