Depending on when you are planing to be, check istocknow.com. It is recommended the you reserve the phone you want, but sometimes you can just buy it without making reservation. Apple store near me has plenty of the sim-free 6S Plus SG available and I went and got one yesterday (return/exchange) due to dead pixel, and didn't even make any reservation. They had plenty in stock as they received shipments on that day. They still have it in stock right now as I speak.