I need accurately cut two by fours. The measurements don't have to be
>precise, at least 1 1/2" thick by at least 3 1/2" wide, but the
>corners/edges must be square.
>
>Any national (USA) stores that stock S4S 2x4s?
>
>I have found one online, a plenty long piece (1-1/2" x 3-1/2" Poplar
>Lumber 2x4) for less than $20 including shipping. That's doable. >
>formatting link> >Thanks.
When you speak of 2 x 4s, you generally mean construction lumber. No, you won't find any with square corners. Most do carry other wood for finish work but usually it is 1" boards.
Your best bet is to contact wood suppliers such as the link you supplied and ask for wood cut to the size you want. You may also be able to get them from a local saw mill, but they may be rough finished.