Try going to the wetlands and near the entrance from loch modan there is a cave with spiders that are easy for your level and there is a ton of tin and another type of metal and they both give skillup.
Darkshore is a goldmine (please forgive the pun) of tin. You'll even find some silver up in the extreme north and you can't swing a dead night elf without hitting a copper vein.
Duskwood is pretty good for tin. You will be moving into iron soon and duskwood has a decent amount of iron as well. Not to mention all the good quests in there for someone your level.
Don't sell the Coarse Stone that you get with the Tin to a vendor. On my server, at least, it sells for more than the Tin in the AH. I sell all of the stone that I harvest in the AH. It doesn't go for a lot, but it sells.