Try the US Ubisoft store
Silent Hunter 4 DLC
Steam has it on sale also, but the best thing to do there is get the collection
Silent Hunter Collection
Their "base" version of SH4 is v1.3, which does not take mods too well, since there are so few anymore that will work on v1.3, but their U-Boat Missions Add-On (works only with the Steam version of the game) brings the game up to the "Gold" level. Plus, you get SH3 and SH5 in the package...