You could make your own pattern with a 36" X 36" piece of paper- freezer paper, tissue, craft, shipping, or whatever you have on hand & can piece together- a pencil, piece of tape, and a piece of string. (I believe most of us learned this as an elementary school art lesson??) Tie one end of the string around the pencil, and make a knot in the other end to stick under the tape. Lay your paper out on a hard flat surface, tape the knot down at one corner of your paper, keeping the string tight, draw the semi-circle. Shorten your string a bit (however wide you want your strips to be plus seam allowance if needed depending on your applique or piecing method) and draw the inside semi-circle. Depending on the size of your rainbow, this can be your complete pattern per color row, a half pattern placed on the fabric fold, 1/4 pattern placed on double folded fabric, etc.
I used this method on freezer paper one year to make a tree skirt, but instead of drawing the inner semi-circle, I made a small one 3" in from the "anchor" corner and cut out the "pannel" to iron to each fabric used in the skirt. (I believe it took 6) It was a very large tree skirt, but we have a large tree and an even larger family. I also discovered this pannel would make a GREAT quilted cape. Anyway, hope this helps. It is really easier than I am describing it... I promise.