Here in Northern California the best (but not necessarily entirely reliable, unfortunately) source where to buy pitaya fruit dried is most likely Trader Joe's market.
Trader Joe's dried dragon fruit product is the pink (the dried fruit actually appears as a very dark purple) fleshed pitahaya variety.
Trader Joe's dried pitaya fruit is imported directly by Trader Joe's from an unknown supplier in Thailand. We have never encountered a problem with quality of the dried pitaya fruit from Trader Joe's, but we all too often find that it is 'out of stock', and we get a variety of excuses as to why. You may be given the opportunity to order the pitaya fruit from them, but often when they do not have it, they also state that it 'is not in the computer system' and that they can't order the dried dragon fruit at all for you to buy later. If you ask them to order dragon fruit for you and they can't or won't, at least ask where to buy dragon fruit elsewhere other than their store. It may make them realize they are losing a customer, or maybe you will get lucky and they can tell you where to buy dragon fruit at another place near you.
A Note about buying dried fruits from Trader Joe's market: Trader Joe's used to be a reliable source of a wide variety of un-processed and un-spoiled dried fruits, but in recent months has started to stock more sugar laden fruits instead of the pure dried fruits. Complain to Trader Joe's about this, ask to order dried fruits from them like the ones they used to stock. They used to have un-adulterated dried gooseberries. No longer. They used to have un-adulterated dried cherries. No longer. They used to regularly stock un-adulterated dried apricots. No longer reliably. The list goes on and on... More and more they are being dropped and replaced (if at all) with sugar coated and sugar saturated dried fruits. PLEASE COMPLAIN TO YOUR TRADER JOE'S STORE SUPERVISOR ABOUT THIS AND HELP STOP THIS DOWNGRADING OF TRADER JOE'S PRODUCTS FROM CONTINUING.
You may have better luck for some dried fruits now at Whole Foods Markets, but they also are not reliable and often have sugar laden dried fruits as Trader Joe's does so much now.