Knot free is just about possible if you specify "unsorted" grades of
softwood i.e. what was in the old days sorted into 1st 2nd and 3rds.
But why would you want knot free anyway - knots are just a natural
feature of many timbers. There are knot free(ish) alternatives to
redwood such as pirana pine etc but they have other drawbacks.

cheers

Jacob