not sure if it's exactly what you want but you could try the Simple Ads Manager
it let's you place ads below every question, you could simply put your source link in it instead....
i'm not sure, but I'm assuming either you need to edit this part in qa-theme-base.php
public function q_view_content($q_view)
$content = isset($q_view['content']) ? $q_view['content'] : '';
$this->output('<div class="qa-q-view-content qa-post-content">');
or need to add it to your qa-theme.php and edit it....
I tried to but i can't seem to get it to work....but it may give you an idea of where to start