eBay will be your best bet, I've had easily a dozen shafts off there that i've experimented with for my sldr. i'm 99% settled on one now (unless there's an oban revenge lurking on here somewhere)
Might be worth searching for the Sldr version, the R15 speeders probably aren't as common (i haven't noticed too many during my searches anyway)
What flex are you looking for? i have the sldr tp 67g x flex shaft doing nothing.
Another option, albeit a last resort, would be to buy a second hand club with the shaft you want, put your existing one in that head and chuck it straight back on eBay.