Still need a little more detail...you want it rough (maybe one side jointed) or ready to use (planed, jointed, etc.)?
Rockler and woodcraft both sell wood online. However really any local lumber yard should carry walnut since it is pretty common.
what you're looking for is 6/4 finished to be 4/4 s2sI'm screwing them together at a right angle to make a simple shelf, so ideally sanded and ready to go.
what you're looking for is 6/4 finished to be 4/4 s2s
Any lumber yard will have this, online shipping is going to kill you, if you dont have basic tools like a joiner/planer and a table saw you're probably going to need to go to home depot or something
X/4 is .25" so 6/4 is a 1.5" board, when you plane it down you take off about .25" to get it flat, so you're left at 4/4 or 1" thick, which is what you want. s2s means that its surfaced (planed, so that its no longer rough lumber) on 2 sidesLearn me what 6/5 finished to be 4/4 s2s means in non-wood speak please.
Hood has a great explanation but here is also some of the other terms (s2s, s3s, s4s, etc.): https://thewoodwhisperer.com/articles/s2s-and-s4s-what-gives/Learn me what 6/5 finished to be 4/4 s2s means in non-wood speak please.