Make sure you're buying from someone who'll guarantee the blank with at least a replacement if turning reveals an internal flaw that is the kiss of death for the stock (which increases in likelihood the fancier you go in blanks), and plan on turning it into a stock relatively quickly so you can capitalize on said guarantee, since even if they offer one, they're probably not gunna honor it if you make it into a stock 10 years from now.
Friend went for a custom stock from Krieghoff. Want to say he chose a 8 or 10k blank. Part way through turning a flaw opened up, so that was the end of the story for that blank, in the dumpster and pick a new blank, start all over again.