Good luck, took me a over a year to find one NOS duct and I am still looking for another. The air filter itself is hard to find too, every now and then I see an NOS one on eBay or K&N still makes one.
I did buy some silicone hose off eBay that worked for temporary ducts, I'l look that up in a minute for you. Either way I'd buy the box when you see one as it's not super common either. Make sure the cap is intact and the seal is on the cap still.
These can be cut down and will fit snugly in the airbox and fits over the carb and secures with a hose clamp. These obviously do not have a "bell" end therefore may not function like the originals in terms of a velocity stack but I did not notice any issues.