I will echo this, specifically at Toronto Fan Expo last month. ES had a couple portfolios of Kubert art that I was offered to look through and it had some really nice art, including a piece I had never seen online that I seriously considered picking up. In speaking the ES employee, he shared that prices were pretty firm due to Kubert(s) not really needing to sell. This led me to believe that is why the art isn’t online – they don’t care if it sells or not.
I also know of artists that bring art to shows that isn’t online. Artists aren’t always the most organized bunch so I would presume that most of them have art laying around that isn’t immediately available online anywhere. Paul Pelletier is one of my favorite artists and I consider him a personal friend – he surprised me with a couple pieces of Hulk art that I previously had not seen.
In summary – you can definitely find art at shows that you can’t online. Not always a lot but the pages are out there if you are willing to search for them.