I bought one through here last spring.
ER- G1064 Gas Exhaust Manifold - Elmer's Repair Inc
At the time, it was the most reasonable.
As far as the tractor, all parts I needed are readily available through Case/IH. Most of mine were aftermarket, although I did get some OEM items.
If you know how to use the Case/IH Ag parts search, you will have to look up parts for the 300B series tractor. I usually copy and paste the part numbers to Google, and look for aftermarket.
You can also use Messicks, a dealer advertising on here to find the Case/IH price, and go from there.
That cylinder may be a toughie..., although I did see a complete hoe unit from a 310 on ebay a while back... Never checked back to see if it sold...
You may also try here:
new used parts for Case tractor, backhoe, dozer, machinery & heavy construction equipment. They have some NOS parts for older machines... I just talked to him last week, a real nice fellow. If he doesn't have it, he may point you in the right direction. He told me even if I just had a question, he is willing to help.
If you need anything specific, PM me, I have quite a few links from where I looked for parts for mine. I have a real good place to get bearings and seals, for like 30-50% savings over NAPA prices. If you don't mind waiting a week to be delivered via UPS.