Rainmeter comes with a few simple starter skins that provide a good place to start learning how to use and modify Rainmeter. There is also a vast and growing community of Rainmeter users who share the skins they create. Listed below are some of the better places where you can get new skins to use and modify. Download as many different skins as you like, you can always switch between them.
Customize.org is also one of the largest and oldest sources of Rainmeter skins. One of the first sites to host skins people wanted to share, it is a good place to find older work that you might not find elsewhere.
Lifehacker Desktop Show & Tell
Many of the most creative minds are continuously posting screenshots and details on their Rainmeter-incorporated desktop setups in the official Lifehacker pool.
The Rainmeter community on Reddit is a great place to get ideas and tips for Rainmeter, see and share things people are creating, and get involved with others using Rainmeter.
Note: As with anything you download from the internet, a certain amount of caution is advised when downloading and installing Rainmeter skins you get from other than rainmeter.net directly.