It's really easy to make your own for any number of fins. This is an old draftsman's trick. Here's how:
Wrap a sheet of typing paper around the tube and mark where it overlaps itself. Put a small arrow showing the direction back to the edge you started from, so you don't get confused. Slide the paper up and mark the other side (half a sheet of paper works well too).
Slide the paper off the tube and lay it flat. Use your ruler to connect the two overlap marks with a straight line.
Measure the distance from the starting edge to the overlap line, to get an idea of the tube circumference.
Choose a whole number greater than the circumference that is easily divisible by the number of fins you plan to use. Say your tube measures 4 5/8 inches in circumference (distance from the corner to the overlap mark on one edge of the paper). Say you want to put 3 fins on your rocket. 6 is greater than 4 5/8 and easily divisible by 3, which is 2 inches. Lay your ruler across the paper with the zero mark lined up at the corner, and pivot the ruler down across the paper until the 6 lines up with the line connecting the two overlap marks (should look like you're going to draw a triangle). Now mark the paper at the 2 and 4 inch marks. Move your ruler to the bottom edge of the paper and repeat. Connect the two "2" marks with a line running to the edge of the paper, and the the two "4" marks. The "6" (overlap) line is your third fin mark. Wrap the paper back around the tube and transfer the marks to the tube, and then use an angle or doorjamb method to extend the line along the tube. Presto! Instant evenly spaced fins.
Say your 4 5/8 tube need 4 fins. 8 is easily divisible by 4 and greater than 4 5/8. Do the same thing pivoting the ruler from the zero mark at the corner until the 8 inch mark lies on the circumference line. Then mark at the 2, 4, and 6 inch marks. At some point your triangle gets a bit 'too tall', and if that happens, switch to a smaller number that's still easily divisible by 4, say 6, which would be divided by 4 to get 1 1/2 inches. Mark the paper at the 1.5, 3, and 4.5 inch marks. Flip and repeat, and connect the marks. For 5 fins, 5 would work well as the number, since it's greater than 4 5/8 and easily divisible by 5 (1). Pivot the ruler til the 5 lined up on the circumference line, and then mark the paper at the 1, 2, 3, and 4 inch marks.
Get the picture?? Works like a champ on ANY tube size and ANY number of fins, and all you need is a sheet or strip of paper, a ruler, and a pencil to do it.
Good luck and hope this helps! OL JR