There's plenty of ships out there, you just need a bit of a luck and some knowledge to find them.
As mentioned above, check the map to see where's the concentration of shipping lanes.
Early in the war you can hunt around UK coast without much opposition from RAF or RN. Check the area around AN26, there's usualy a lot of single ships there.
Another good spot is AM53. There's a good chance you'll find a convoy there, and a lot of single merchants.
On the south there's a lot of ships entering the channel between England and Ireland. Further southwest is a good spot to intercept convoys (BE36-BF17).
You mentioned the AL sector. You probably won't find a lot of single ships, but can spot a convoy or two.
Maybe you should take a more active role while searching for the ships. Every few hours dive and personally listen for the propeler noise. You can hear ships 30km away, while your sonar officer will rarely detect anything beyond 20km. On surface, the watch crew can see a ship from no more than 16km, if the visibility allows, and you have GWX 16km atmosphere mod installed.
Good hunting!
