No Iron and Metal?

Since you said that your Jackdaw isn't that strong yet, the best way for you is probably taking out schooners and low level brigs. As you have already mentioned, it can be difficult finding ships with metal, but they do exist, promise!

I found that the hunters which come after you when raising your wanted level are a good source of metal.

In your circumstances, I would do the following:

1. Raise your wanted level to level 1, this will cause a level 17 brig to spawn and hunt you.
2. Incapacitate the ship and board it to receive the full quota of loot. Repair the Jackdaw if it gets damaged.
3. Fast travel to another location. This will cause another hunter to spawn.
4. Repeat steps 2. and 3. until your wanted level rises to level 2.
5. A level 20 brig and a level 8 schooner will spawn and hunt you.
6. Incapacitate both ships and board.
7. Fast travel to another location.
8. Repeat steps 5. to 7. until your wanted level is level 3 or close to it.
9. This is probably where you want to decrease your wanted level. Best way is to board a ship and lower it by one. But if wanted level 3 ships spawn and you don't think you can handle them, you can always bribe the local officer to reset it completely.

I find forts are the best places to fast travel to. They have a harbor master (in case you need to repair, sell loot and so you can upgrade whilst farming) and a local officer (for bribing) right next to the dock. The disadvantage of a fort is that you will have to fight away from it. If the fort sinks the ship, you won't be able to board it, nor will any loot spawn. However, an advantage of being so close is you can run to the fort when things go bad.

Try to always board over sinking it outright. It may feel like a chore, but schooners and brigs can mostly be cleared just with the swivel cannon. You also get the full loot instead of half, and a chance to repair the Jackdaw or lower wanted level (or keep the ships if you need them).

Make sure you sell the rum and wool (and any extra wood or cloth you aren't using, as you don't want your stores to fill up and waste loot).

Hopefully that helps you with your lack of metal. When you upgrade sufficiently, you can farm higher wanted levels and therefore receive more loot!

Aside from plundering Warehouses, the best way to get Metal is to raise your Wanted level. Your best bet is to use the telescope and to see which ships have Metal--even if it's a little bit--and destroy and loot them. Eventually, if you repeat this, you can raise your Wanted level enough to bring on the higher-leveled ships which will normally have a large amount of Metal you can loot (if you destroy them).

And worst comes to worse, if you feel like you won't be able to destroy the stronger ships that might be heading your way, you can always lower your Wanted level and repeat the process over again. Or get desynchronized and get your Wanted level lowered automatically (which if I remember right, shouldn't remove the Metal or any of the loot you've managed to earn).

Do head South as well. There are larger ships (more likely to carry metal), although they are harder work to take. Use the spyglass to check the contents first so you can focus on the ones you need. If there is a group you can use the smaller ones to repair mid battle, which will let you take on larger opponents than you should be able to.

You can also get small amounts of metal from random pickups at sea or islands - this is a good way to get the cheap first level upgrades to allow you to take on the harder ships.

Lastly, do the trading minigame to raise funds quickly - once you have a few frigates or Men o' War you can raise a lot of money fast, which is needed for some of the later upgrades.