www.HondaCB360.com has the intake manifolds. the site is run by Zephyrking, he posts a bit on this site.
They are not THE cheapest, but they are high quality and he ships fast. Her's located in Austin Texas.
He also has a bunch of cb360 Specific parts. I've bought many of them. The quality is good, shipping reasonable, fast and personal service.
I delaminated my rear brake shoe, and needed ones quick. Sent an E-Mail at 10 PM. The next morning they were on the way, before I even paid for them.
If you are looking for only the cheapest price, then you may not find that there, but if you want the great service, Quality parts (Brakes are EBC for instance, not Chinese Clone Parts) and want to be sure you are getting CB360 specific parts, I highly recommend them.
Zephyrking is a stand-up guy. Tell him I sent you...