I would take a bra that fits the breast I had, or still have, and cut it away from the bra leaving at least half an inch where I could. Stuff it a bit with some fiberfill, and carefully pin it to a backing fabric along the actual cup edges and trace the shape on the backing fabric add your 1/4" seam allowance with the pen. Remove the cup piece and mark the center of the 'cup cone' with a pen, and draw a line to the nearest edge. Carefully cut the line you just made so the piece lies flat, pin it onto a piece of paper or fabric and trace the edges, add the seam allowances including in the 'v' cut area. You now have a pattern you can fine tune to fit you. Make the forms out of pastel or white (dyes can irritate tender skin) cotton, rayon, silk, or other natural fibers so the breast form does not sweat or stick to your skin. You could even add a small water balloon inside with the fiberfill to approximate the bounce so you feel more 'even'. If you want to use a balloon, you can leave the form as a pillow cover sort of thing that can be washed between wearings, and the balloons can be replaced as needed. We did this for my friend who could not afford the (very expensive at that time) breast form. We made her all kinds of colored covers for her "balloons", and found a drinking cup the right size to use as a guide for keeping the balloon about the right size. If it fits in the cup, it will be nearly the same size each time. We filled the balloons with distilled water to avoid bacterial growth inside them. Good luck with this I hope this is helpful if you can not find a form pattern. Back when we did this, there were no patterns to use.
I am "the McGiver" of the fabric world!