The dupe I came across is the Rimmel 60 Seconds Nail Polish in "Sage All The Rage". This shade can best be described as a muted mid toned sage green, as the name suggests. It leans slightly cooler in tone. This is available year round in the permanent collection and retails for $1.50, what a steal!
OPI's "Thanks A Windmillion" is a muted mid-toned sea foam green with a hint of blue in it. The color is more blue and just a little darker than "Sage All The Rage", but they are both a muted color and not overly bright. A full review for OPI "Thanks A Windmillion" can be found HERE.
Here's a comparison of the two. (They look almost identical on the nail!)
|(Left) Rimmel London "Sage All The Rage", (Right) OPI "Thanks A Windmillion"|
So if you can't find OPI "Thanks a Windmillion" or don't want to spend $9 on one polish, go grab "Sage All The Rage" by Rimmel!