Welding the crack is fine if done correctly. Thousands of us TR owner have had to do it. The only TR without a crack is the one not driven
There are no aftermarket headers and used ones run $700+ when you can find one (usually once a year).
I believe Steve Kaminski was thinking of making them because he does a lot of turbo 3.8 swaps into other f-bodies. I have not heard anything else, but I don't think the demand is there.
You can get a custom set made, but that would be seriously expensive.