I'd just get an lvl beam. Stable, warp-free, maybe expensive. Those are the laminated beams used for free-span constuction.

Pressure-treated won't work...like the other post said, arrives wet, dries warped.

You could also laminate several layers of exterior grade 3/4" plywood. Have the lumberyard rip a bunch of 4x8' sheets into 8" wide strips, then laminate using wood glue and drywall screws, staggering the seams.