Thanks, yeah not through Yamaha. Bike Bandit       www.bikebandit.com/oem-parts/1975-yamaha...0b/o/m7094#sch221477  has it. 

Bike Bandit has access to genuine Yamaha parts, but I seriously doubt it is available new from Yamaha, I see where it says available and ships in 1-2 days, but if you click on any part for that bike it says it is available and ships in 1-2 days, as an example click on the fuel tank, crankshaft, carburetor, complete fork assembly, rear shocks, frame, swing arm and it will say available and ships in 1-2 days, and we all know 100% for a fact those parts are no longer available new from Yamaha. The only new manifolds I have seen are an occasional N.O.S. for sale on eBay or the aftermarket one I mentioned earlier. My guess is you can try and order it from Bike Bandit and it will not let you pay and check out due to it being no longer available or it will let you pay and check out and take your money then later cancel your order and then refund your money due to it being no longer available again, or maybe they have a secret hoard of parts for that bike, hopefully if it works out for you report back as I am sure others will want to buy new ones at their listed price, either way good luck.

Hot foot moto is about the only place that makes them I think.  There used to be a seller in the UK that had nice ones for around $50 but sadly they went out of business a few years ago.

Probably true on availability, a lot of stuff is unobtainable right now,
I am considering getting one off of that auction site, there are decent
ones from time to time. However, almost always listed as "has cracks",
if not all the way through (like mine is) I may try to get as decent one
as possible, and perhaps coat it with an epoxy to get the most miles
i can out of it...previous fix was a thin inner-tube rubber strip wrapped tight and
zip tied around manifold. 
Has to be a common problem for slot of us ? 
Couple of pic's..... I've had this for 10 years, it's just like the one I had
in high school that I always regretted selling. Changed out the fenders
and couple other cosmetic things. It runs and sounds very good and
of course, it's a blast to ride.