Is there a reliable, continuous source for dragon locations?

Even though you can kill their guardians and get a Cleared Map tag under the location, exterior Word Walls will eventually respawn, both Dragon and Chest.

If you crisscross across Skyrim via Fast Travel, you should get more than enough Dragon Bones / Scales from previously cleared Word Walls that have since respawned.

If you're still looking for more dragons, you can still use the Word Walls, just wait for an in-game week (or however long it takes) then repeat your circuit across Skyrim.

You could use the console if you wanted to, command: placeatme. Or just run around a lot.

The radiant Companions quest, Dragon Seekers will have you killing dragons. This is also the case with one of the random radiant Bounty quests (a quest given by an innkeeper, jarl, or steward), and with the radiant Blades quest, Dragon Hunting*.

Note that radiant quests are repeatable and randomly generated, so the quests above are a good, continuous source for dragon locations.

* Note: If you have received the "Paarthurnax" quest and have not finished it, then you will not be able to do the "Dragon Hunting" Blades quest, as well as any quest that the Blades (Delphine and Esbern) give or has given, until you finish it. See the "Paarthurnax" article on UESP for the repercussions of finishing or not finishing that quest. Those playing on the PC can install The Paarthurnax Dilemma mod to get around this issue.