You can go to John Deere's website and download the manual for your B in PDF.
From there you would have both the number on the bearing to cross and the JD number in the book. John Deere didn't make the bearings, they should have a number from who did. A parts store should be able to cross them pretty easily.
It is amazing what JD still makes for the older stuff, and usually bearings are priced very competitivly with a parts store... and they are really good bearings.
Go here to look up your tractor, on all of my computers the older stuff in PDF loads slow off of the internet, so go ahead and save it to your computer once it comes up for future referance. It will open as a white box at first, give it time and it will come up. I have a flash drive just for this at work, my own little service manual shelf in my pocket. For looking up your own stuff you can't beat it.
http://jdpc.deere.com/jdpc/servlet/...vlets.TopNavServlet?irand=2674278012977505504
Type your number in here to see if it subs up to a newer number. If you make a free account you can see pricing, availabilty at your local dealer and usually a detailed discription. Also above where you enter the number you can hit "non JD part number" and it will sub it to a JD part number if does. They were pretty unoriginal back then and a lot of different company's parts like bearings and seals sub around across brands. Usually a good guy at a parts store can sub to their number from a JD number.
https://jdparts.deere.com/servlet/com.deere.u90.jdparts.view.publicservlets.HomeUnsigned
I am showing a JD7656 for the inner race (with the rollers and retainers as a seperate part as well as the outer race) which subs up to a JD7654 as a complete bearing and replaces the three seperate parts of the orignal. That lists from JD for $51.00. My local dealer doesn't have them but a satelite store does, so I could have them by noon tomorrow. The parts book only shows 2 required for the fanshaft itself.
If you don't have anything to go by, get a IT manual at least to help you set it back up after you have it all apart.
I have a '46 B, pretty much the same as your '39, good little beasts.
