The big box boys would have to spec order, I think, not sure they could even do that , might be outside of their big box system :-/ :-/ My local yard only carries a few 20' ers , for a job like yours I'd go to a good yard and order them . It shouldn't a true "special " order but 20 ft-ers don't just fall off the truck , well they could but ya know a lil joke .
[highlight] They would be uncommon / not sold every day / rarely called for item, so ordering them would be best [/highlight]. The supplier seem to do a better job storing lumber , or they move so much of it that the lumber doesn't lay out in the weather, suns the worst thing on lumber around here. More stock is lost in summer due to quick irregular drying than in the other season we generally have which is called spriall (spring / fall) although for the past week we have been transported to Nome, Alaska
So back to your lumber shouldn't be any more costly , other than it will be more total brd. footage , than your standard 2 x12 stock. And you'll end up with better looking / straighter stock MTL
YMMV depending on the current supply they have on hand .