I know that you don't (can't) purchase strip stock in the length of the boat you are building. But what is the best length to buy? 8'? 10'? 12'? or a mix of several lengths?

It's true that regular lumberyards and box stores won't often have longer boards, but some 'specialty' lumber dealers that sell to wooden boat builders will. The last time I bought boards for ripping strips (several years ago) the only clear cedar I could buy was in 22 foot lengths. So it pays to ask, if you have a specific length in mind.

The availability issue is entirely separate from the optimum strip length issue. It's common to use less than full-length strips, but I always found it helped to have full strips along both sides of the the sheer line, at least.

You can always make them shorter (cutting) or longer (scarfing) so it's just a matter of labor vs. cost/availability vs. convenience - swinging 20 foot boards or strips around is awkward unless you have a big shop.