FWIW, I recommend the reddish/brown acq pressure treat lumber for structural components instead of the green pressure treat. It'll keep a nice color for a good 7-8 years and takes paint and stain very well after that.
For some reason the local home depot/lowes characters like to put this wood out in the landscaping/nursery area instead of the lumber area.
I've been able to find it in 6x6, 4x4, 2x4 sizes. I havent seen a source for 4x6's but I havent looked that hard.
Be fearful when others are greedy, and greedy when others are fearful. Just another form of "buy low, sell high" for those who have trouble with things. This rule is not universal. Do not buy a 1973 Pinto because everyone else is afraid of it.