I just got into the game and have been tracking down copies of each of the Chapter Packs. I've been able to find them all, except for the "Secrets and Spies" Chapter Pack. Is this one usually harder to find than the others? Thanks!

the whole of that cycle of packs is out of print, and being re-issued soon in the new format. you'll probably find copies lying about here and there, but it's best to wait for the reprint

City of Secrets and Tales from the Red Keep are impossible to buy at that moment. Those two are the best of the cycle and maybe even from whole list.