Going through list of plugins not active on my blog, I saw the Q2W3 Fixed Widget.
I remember some time ago, I wanted to make a particular sidebar widget fixed when the page is scrolled down. After some googling, I finally settled for this plugin.
This plugin adds a Fixed widget checkbox at the bottom of every widget. If the checkbox is ticked, that widget(s) become fixed.
Mind you, the plugin has a configuration page. I assume you know CSS. Setting it up shouldn’t be a headache. You can get the setup info from the plugin author’s website. If you had any problem, let me know via the comment.