The two big shuttle companies are:
Gray Line Costa Rica -
www.graylinecostarica.com - and Interbus -
www.interbusonline.com
Interbus appears to have a 2:30 shuttle to Arenal/La Fortuna. Neither calls at SJO airport itself. You'd probably have to get yourself to one of the hotels across the highway, such as the Hampton Inn.
Both are good for getting you around Costa Rica.
I have no experience with Arenal.net