Greta go here for how to fix your fridge:
Refrigerator Defrost Cycle Diagnostics
Go here for the cheapest and most complete parts:
http://www.***********************.com/default.aspx
I just fixed mine and I put in the thermostat and timer and what it needed was the defrost heaters. It isn't hard. You need to take off the panel in the freezer behind the shelves. The first link has the pictures. To see the defrost heaters there are two metal panels you need to take off and look at the heaters.