where to put third party library

I would like your opinion on where to put the third party library I’m using.

I’m developing a plugin that will generate an excel document, for this I will be using phpExcel. My first thought was to put the phpExcel library in a subfolder to the plugin. However, I will need to use phpExcel in another plugin as well and that rises the question on where to put it. I dont want to use wp-includes since I feel this is for core-files, the plugins folder is for plugins and this is not a plugin itself. Should I put a “third-party-libraries” direct in wp-content perhaps? Of course I could do whatever, but I wonder if there are any “Best practices”?