I think someone on here had an extra set of Brad's custom 1MZ headers they were looking to sell. But that was a year or two ago. You could also try ToyotaNation or SolaraGuy, if those boards are still active. Or whatever the equivalent is now on FB...
Really, the OEM manifolds work pretty well as long as you improve or completely replace the OEM Y-pipe. The OEM Y-pipe's flex section (after the merge) necks down to about 1.5".