Where Do I Get Social Media Buttons?
They usually come in multiple sizes so you can use the size that fits best with your blog. Here are a few sites that have pulled together a bunch of collections:
33 Beautiful Social Media Icons Sets for Designers and Bloggers
Huge Collection of Social Media Icons
44 Must Have Free Social Media Icon Packs
These sets come in an EPS version which you can use in a vector program like Illustrator. They also have PNG files.