Place chicken breasts in the bottom of the slow cooker, and top with all ingredients EXCEPT butter, flour, milk and sour cream.
Cook on high for 4 hours.
Shred chicken (this might be easiest by taking the chicken out of the slow cooker, and then placing it back in).
In a small sauce pan, melt the butter over medium heat.
Add the flour and whisk for a minute or two until it's bubbly and smooth.
Add the milk and continue to whisk until it's bubbly and thick, then add it to the slow cooker.
Add the sour cream to the slow cooker and mix it in well.
Top with crushed tortilla chips, shredded cheese, avocado, and fresh cilantro.