As mentioned the cabanna's are only at the GF, CR, and Poly. You have to be a resort guest at that resort to rent one. They are located at the pools, not on the beach area.
This info is from the GF thread:
Cabana Rental:
Rental Cabana amenities include:
. Two padded chaise lounge chairs
. Two padded straight-backed chairs
. Plush towels
. 22 inch LCD television with cable access and optional earphones
. Ceiling Fan
. Mini refrigerator stocked with complementary sodas and water
. Complementary fruit basket
. Private dining and poolside massages are available at an
additional cost.
A half day is $110.00+ tax, and is from 10am - 2pm OR 3pm - 7pm
A full day is $185+ tax for 10am - 7pm
Credit card needed to reserve; 24-hour cancellation policy; weather-related cancellation less than 24 hours, no fee, but any other reason and your CC will be charged.
Call 407 WDW-PLAY to reserve.
* I don't know the prices for the Poly or CR cabanna's but I would assume they are the same as the GF.