Plow bottoms are in a separate parts catalog
here.
I think what you refer to with "plates that attach to the back of the moldboard" are called moldboard extensions- key #1 on the page I linked above. I'd be very surprised if they're still available from Deere and you probably don't need them. I would advise full-cut shares.
You may, however, need coulters to do a good job. The wire trick from another poster is a good one, though I've never tried/needed them. Without coulters to attach the wires, weed hooks might help- it's the same principle- illustrated
here. Probably you would have to make your own, but they don't look too complicated.
Don't mess with trying to find/use jointers, and of course they only work with coulters. I've never seen them anywhere but a scrap pile. Another tip: Your next pass after plowing should be traveling in the same direction. Going the other way could tend to lay the cut slice back over as it started.