I found an area with respawning convoys. I have at least 10 convoys from this spot. Much easier than getting intel from taverns, but you have to watch your wanted level. I can also confirm that you can sink the convoys and do not have to board them.
Fast travel to Lewisporte (North Atlantic 722, -379). Look at your map and to the west of you , or southwest of Harbour Deep (North Atlantic 592, -347) and you will see 3 ships close together (one frigate and two gunships). There is a Templar map location, at (635, -446). This cove seems to be where they respawn at. It will not automatically appear as a convoy, but you will see 3 ships close together. As you approach, you can either spyglass it, or your crew will notify you of the convoy.
I kept fast travelling between Lewisporte and Harbour Deep and they kept respawning. If they didn't, I fast travelled to another map (back to New York or River Valley maps) and kept coming back to this location, or Harbour Deep. Also, when your wanted level gets really high, go take on othe missions or activities, while your wanted level resets over time, or just desychronize, since you probably already know that you can't lower your level with bribes or boarding ships, like previous games (LAME).
Repeat this until you finish off the 20 that are required.
Note: Sometimes when you travel, there will be more ships close by, other than the convoy. It is best to have the Morrigan upgraded, but always remember that you can board a ship to repair the Morrigan
There have been reports that this issue has been patched with the newest DLC update. Gabwe posted in the comments another way that he was able to repeatedly find convoys:
1. Fast travel to any viewpoint in New York
2. Fast travel to Anticosti
3. Fast travel to Fort Louis - there was a convoy almost every time
4. Rinse and repeat.