We can roll threads onto blanks, whatever length you like up to 8" and generally much cheaper than stock bolts. Let me know if you need some made and I'll see what blanks we've got in stock/order if necessary.
We were also looking into doing titainuim bolts, again with rolled thread. Titainium is (basically) twice as strong, half the weight, doesn't cause electrolysis and doesn't work harden (and then snap) like stainless. Cost would be about the same as a merc stainless bolt.