How to Ensure Straight Fretboards

I am a hobby dulcimer builder. I have been trying to resaw and otherwise prep my own wood for last few builds. I am having a hard time getting my fretboard stock as straight and true as it needs to be. Even with joining and thickness sanding, some pieces still tend to have a slight bow. Is this just a matter of wood not being dry enough to start with? Do fret boards need to be made from quartersawn material? Thanks for any advice.