The value pf 15K isn't very critical, it's simply a handy value vapers have adopted. As long as you properly insulate the leads, the number of resistors in series isn't important other then the extra space they all take.
I'd offer to sell you some 15K resistors, but we do all of you shipping in padded mailers so have to charger $2.34 to ship 50 pcs of resistors sold for $0.99