I found the solution to my problem! Hope this may help some others on this forum (I just joined last night, this is a great forum, very informative). Anyway, the answer is this: Takeoff Destination Service S.A offers daily flights from SDQ to El Portillo (my question was, technically, how to get from SDQ to Isabela, because I (incorrectly it turns out) thought that all flights to El Portillo take off from Isabela). But my end objective is to get to El Portillo. So this looks like a great solution. No bus or taxis from SDQ to Isabela, I can fly direct from SDQ. The website for Takeoff is
Home. They have many other domestic flights also. The cost from SDQ to El Portillo is $59. And thanks to the 2 persons who posted replies to my question.