I have the Denise interchangeable circular knitting needle set. By using the 2 extenders in the kit, I could join the 19", 16" and 14" cord for a total of 49" plus the two 4 1/2" needle heads. That would make the circular length a total of 58". There may be additional extensions for sale to make the cord even longer, as there are still other cords available in the kit.
These kits are sold in most local yarn stores. You could also buy 2 kits to make an extra long length.