mkia9 is right so far the exclusives have turned up in a variety of different ways but never within the single pack purchases.
I so need to get these on the New thread...anyway so far its been:
F35 -Apple Jack
@ Canterlot Gardens
@ Hasbro Pop-Up Shop [during NYCC]
F36 - Pinkie Pie
@ Collector's Box [from Target]
F37 - Rainbow Dash
@ Comic Con
@ Hasbro Pop-Up Shop [during Selfridges Toy Department launch]
@ NY Comic Con
F38 - Fluttershy
@ EverFree Northwest
@ Enterplay [given in response to online survey]
F39 - Twilight Sparkle
@ Collector’s Tin [all stores which carry FiM trading cards]
F40 - Rarity
@ Bronycon 2012
F41 - Special Pony
@ NY Comic Con
F42 - Octavia
@ Collector's Box [from Hot Topic]
Apologies if I have missed something from the list