Hardwood lumber is a raw material and it will have flaws.  You may be able to get almost “clear” boards, but there is no guarantee that any lumber will be 100% knot-free.  Lumber grades are determined by the National Hardwood Lumber Association and the highest grade is called FAS.  FAS means the boards have to be 83% or more clear on one side.  There is always a “waste factor” in woodworking unfortunately, because in most projects you are cutting your parts from boards and arranging them for color and grain.  Many times you can either flip the board or work around knots when cutting out pieces for your projects.