Couple of thoughts, you might try this place thou I'm sure there's a ag. equipment wrecking yard closer.

Farmland Tractor Supply. Tangent Oregon   877-928-1646  They do ship ups, parts are about 45-50% of new.

The other idea is there is not that many actual different "brands" of knotter systems, maybe 2-3  I think. Could be, that a New Holland or JD. or CNH or Freeman baler may have the same needle ?  (different color paint,, same manufacturer?)

It would be pretty close to impossible to reweld the needles accurately enough. You would need a really good jig, prevent warping, maintain exactly the right arc of travel,  etc.. Then the whole needle would still have uneven tempering, and may not run exactly straight within that 1/32 inch or so thats required.

I checked needle prices (curiosity) 3-4 months ago,, $600.00 per needle new. (for Freeman twine baler,, lol probably $800 now)

Another idea,, if you can find the whole knotter assembly, and needles off a twine baler and switch to twine.

 Sorry no great answers but least a start of where to look, or ideas.