It's a hard one. My mum used to say "Are you sure you have looked properly?" Then promptly find something I swear wasn't there when I looked. So first thing I would suggest is double check, then check again :)
Once you are sure you have done this think from the users perspective be imaginative.
Links are adverts to your site so you need to make sure you put them in the right place but you might be thinking too small. For example your field no matter what it is must fit into other bigger fields so look in dmoz I would say and identify you catagory then go one higher and see if there are any sites that are a good fit for partnering with. This keeps it on theme so you get targetted advertising but also gives you more scope of sites to look at.
Hard to imagine that you've seen EVERY possible web page in the world related to your site.....broaden your perspective....if you have a widget selling site, then look for woodget selling sites that sell complementary products....or widget cleaning sites that can enhance your product....or hotel sites in the widget city area....there are tons of paths to take.
It’s time for you to start moving outside of your industry. Do you really think the search engines can tell if a link back to your site is in your same industry? Google can't even seem to tell if you’re linking back to yourself.
I have seen a site with one PR9 link from a site in a completely unrelated industry bump it to a PR7 overnight.
Now, if you’re looking for sites that bring value other than PR, think about what else visitors may be interested in and get those links. For example, if you sell computers, how about office supply sites? Or computer books? Or office furniture etc…