"Tackle Opportunity" is a term that I began using last season in the Forum. There may be other services doing something similar at this point, but I don't know what they'd be calling the metric, if it's available. We have a spreadsheet with an expanded set of data thanks to Larry Thomas this season for subscribers -- weekly numbers broken down by the metric's component parts, but you can do many of the calculations yourself.
The formula was posted in this thread from 2007 -- it's simply rush attempts plus pass completions plus sacks, which encompasses all plays from scrimmage that resulted in a tackle. It's not perfect. A small percentage of plays will generate two opportunities for solos or no opportunity for a solo. And it's limited to total team opportunities, since, for the most part it's very difficult to isolate a "defensive target" on each play.
You can copy the relevant stats from any number of internet sources into a spreadsheet and input the formulas yourself without taking an inordinate amount of time. That's what I did last season before Larry greatly expanded and improved the process.