Agri supply is my go-to for disc harrow parts:
Search Results | Agri Supply(R)
Make sure you aren't getting too much money into this one. A good used tube frame harrow should be 500-800 dollars and will be way better and heavier than this one.
If you only need it for a one-time use, use it like it is and sell it for scrap. I did that with a disc my dad gave me and sold it as a pile of crap for 100 or 150 bucks to someone who thought it would work for them.
Not trying to discourage you, but I think you are in for hours of agony with a torch and penetrant trying to get everything apart and back together. Then a bunch of money for blades, hardware, bearings, etc. And a tool that isn't all that great when you finish. If you are doing it for fun or for the satisfaction of having a 'new' old disc, then go for it. I totally understand the appeal of that approach.