You'll find a few Tin nodes in the Barrens yet. The Stonetalon area is an option. Windshear Craig also has some Tin. You're likely still a bit low for the Charred Vale dependent upon how you're geared, but that's another resource to consider. Copper will take you quite far.
The Silver, Gold and Iron are going to be found in higher level areas. I do believe you'll survive fine on the west side of the river at level 22 which will provide Tin and some Silver. With a mining skill of 150 my alt is still skilling up with Tin, though it is green now.