You can slap on any left crank that will fit the BB. It won't match, but you'd have to buy a full crankset if you can't find the left side only for 7400
I'd start looking in
sports > cycling > road > parts and accs. > cranksets
You can search for DA, Dura Ace, 7400.
Throw on something that works for now and save the Dura Ace 7400 search on ebay and it will email you when something comes up. Worked for me YMMV