sk3 213kv is in stock, as is the 260kv. yes, most people say 245 is a good sweet spot, but both of these are perfectly viable.
for 6s I'd say anywhere between 200 and 300kv is perfectly fine.
http://www.hobbyking.com/hobbyking/store/__18180__Turnigy_Aerodrive_SK3_6364_213kv_Brushless_Outrunner_Motor.html
http://www.hobbyking.com/hobbyking/store/__18127__Turnigy_Aerodrive_SK3_6354_260kv_Brushless_Outrunner_Motor.html