I found Semper Fi from Amazon as a download which is currently down, however, I sense that you want a physical copy of the game butI DO NOT believe that there is an actual RETAIL (physical) copy of Semper Fi AFAIK only download versions are available.
Code:
http://www.amazon.com/Hearts-Iron-Semper-Fi-Expansion/dp/B003P2VTAU
You can get Semper Fi only on Steam for 5 bucks (think its only for today, regular 20) but I think you require HOI III on steam. Or you can buy the whole package today for 14.10 including the extra sprites which is still less than the retail price of Semper Fi. Just offering some options.
Code:
http://store.steampowered.com/app/42900/
It is 50% off right now on Gamersgate for $10.
Code:
http://www.gamersgate.com/DD-HOI3SF/hearts-of-iron-3-semper-fi