Unfortunately, I have no personal experience with long-term rentals in Flamingo, as my trips are usually only for a few days, and my rental condo's are an hour North - in Playa del Coco. But I might be able to get you pointed in the right direction.
I've seen many signs along the road into Flamingo, in the towns of Huacas, Brasilito and Playa Portrero for rentals, but I think most of them are owned by locals or foreigners with local management. Your best bet is to either go there and drive around. Once you're in-town, you should be able to find something suitable.
The other option (if you want to find something ahead of time) - would be a local realtor / property manager. There's 3 or 4 real estate companies in Flamingo who rent homes / cabinas for weekly & monthly rates. You can do a google search and find the local companies. I know there's a branch of Century 21 within 50 meters of the entrance to the Flamingo Beach Hotel, and another company within 100 meters of the hotel.
Below are the websites from the 2 companies I mentioned above, located in the town of Flamingo Beach:
http://www.century21costarica.net/
http://www.flamingobeach.com/index.html
...And here's a few more websites to search for more rentals in the area:
http://www.flamingobeachrentals.com/index.htm
http://www.villascostarica.com/indexfl.htm
http://www.costaricarentals.biz/
This should be enough to get you started. Good luck, and have fun!!!
MG
