I can't comment if there is a conventional way, but my thinking is that it really makes sense to use the libraries folder for this purpose and I have seen others doing the same.
Personally I have used the libraries folder in the same way, and later on my extension I usually load the library with the jimport function.
Like this for example:
jimport('simplehtmldom.simple_html_dom');
it will include the php file:
"/libraries/simplehtmldom/simple_html_dom.php"