Hi Ed,
Congratulations on your trip. Atenas is a nice spot, as is Arenal. Lots of rentals in Atenas but I’ve heard that recently the rental market has been tight in Arenal because of limited inventory. Here are some sites to check out:
Atenasbestclimate.com
Airbnb.com
Vrbo.com
Moranlakearenal.com
For airfare, I always check Kayak.com for good deals. The sooner you buy your ticket the better, of course. But Jetblue also has cheap flights and I find they have good service too. Plus your first checked bag is free!