Here is a great set of twitter avatars for free use from Ape Lad.

And here are a 100 more for your inspiration.

This free set includes 50 icons of the most popular social media networks on the internet. The icons are designed in 32px and 16px vector format. With the vector format, you can scale the icon to any size to fit with your design or use it in high quality print materials. Here’s the download Link.