A/UX was discontinued a long, long time ago. It has not been sold for at least 4 years, and possibly more. If you are determined to run A/UX, you will have to search for a used copy. Try ebay.com or other auction sites.
However, I would instead recommend you look into several of the free Unix operating systems available on the net:
Both will run on 68030 and 68040 machines, including your Performa and Quadras. I ran OpenBSD on a Quadra 700 for several years.