While ETO isn't super hard to come by, getting to 100,000 seems tough. One thing I found that has worked well so far is street performing. You will need to find someone with the "Living Statue" ability regardless, so I found the person, and then just left the game running with the operative standing just off a busy walkway doing the living statue ability. ETO accumulated surprisingly fast. Most people who walked by tipped the operative 5 ETO. It comes out to roughly 4,000 ETO an hour.
A great spot to do this is in Southwark. Go to the Old Kent Road fast travel point and go into the roundabout. Jump over the low black, angled wall and stand behind it so people don't run into you.