I've got a handful of hand and power tools for working with wood, but notably, I do not have a miter saw or planer.
I've only ever built mostly low quality creations, though I posted here when I worked on some gates; they came out pretty nice and are holding up well. My point being, I've accepted the mediocre quality of "big box store" lumber for these projects. (In my case, Home Depot.)
However, for some deck boards that I'd like to replace, I'd assume I'd have to go to a more posh (!) small lumber supply store / sawmill. I imagine they sell mostly rough, but higher quality lumber - then you cut, size and plane it to your needs. Is that a correct assumption, or do they sell "ready to go" stuff, too? With deck boards, I obviously need something that is the correct width, length and thickness, and won't readily warp. If I measure exactly what I need, can I get that from a lumber source, or do I need to get spendy and buy a planer?