One of the places that sources some parts for South Bend lathes is Miller Machine and Fabrication:
I've dealt with them before and was very well satisfied. If you need something you don't see in his product list, call and ask anyway - he might be able to make it.
BTW - on the South Bend lathes, the #1 thing to make sure you maintain is your lubrication. The reason why things like the half nuts wear out is that the past owner(s) of the machine didn't lube or clean off the lead screw. Way oil would be good lube on the feed screw, because way oil has tackifiers to keep the oil on the surface.