Your best bet is going to be Ebay. The CX500 was a popular motorcycle, but only produced for a short period. This motorcycle was also known as a Honda Silver Wing, and fortunately they both have inter changeable parts. I also recommend finding the carb mounting which was prone to cracking.
The easiest place to find any motorcycle parts is online. Check motorcycle parts catalogs for modern carbs that will fit your bike. Ebay may also have original version but that are not always in the greatest shape and shipping can be a hassle. Also check swap meets.