Be a Deviate

Here's a good way to make some easy WoW gold from level 20 onwards.

Perfect Deviate Scale is selling for around a gold each on most servers. This is needed for the Deviate Scale Belt in much demand for level 19 twinking.

There is only one place this drops and that is Wailing Caverns. The mobs outside of the instance range from 15 to 17 elite. They drop Deviate Scale and Perfect Deviate Scale. If you can skin you will gather a lot more scales, both regular and perfect, as well as light and medium leathers.

The regular Scale isn't worth a lot but still sells for 15 to 20 silver or so. The mobs inside the instance will also drop the scales but I haven't found the drop rate to be any higher. If you are Alliance on a PvP server, then the instance is a good idea to avoid unwanted agro. For PvE servers you can stay in the caves. The mobs spawn fast enough to hunt continuously.