I think they only come when there's a 3* esper update from the bingo challenges, why did you use some on cactaur tho
note to other reply, he didn't say rainbow broadstones tho its understandable... should probably call them "sourcestones" for less ambiguity
i think so far there have only been 4 or 5 rainbow stones available total. 1 in the regular shop, 1 in the paid shop (in a 79.99 bundle), the free one from anniversary, there's one in the FFX trial list 4, and possibly one more as another reward from somewhere I can't recall. More will probably come as more events and units get EX'd.