I did a lot of research for larger cup sizes for my book. You will see some reviews on Amazon from women who have made bras for large cup sizes using my book, see How to Make Custom-Fit Bras.
I am not surprised you have trouble finding bras. And I am pleased to tell you that my approach to bra making starts with a non-sewing technique where you start by making a mold directly on your body. Once you know the shape of your body, you make the patterns, then a trial mock-up using minimal sewing, straight and zigzag stitches. No messing around with generic patterns that you then have to try to adjust to your specific body. It makes me so happy when I get comments from women who tell me they have made the most comfortable, supportive bra they have ever had.