Where to find hidden GreenManGaming coupon codes?

Is this this one?

newsletter?

I don't think there is a 20% off of any title right now. There is only a 20% off of 10 (or so) specific titles.

Edit : The usual 20% off (not on specific titles) are usually on their main page.

They aren't secret and they don't hide them lol
Sign up for their newsletter and they regularly send you voucher codes via email.
Users also post all known vouchers in this thread on the Playfire forums.

there are no secret coupons. since playfire you can find them on the blog , just scroll till you find a voucher, but theres also the possibility there is no voucher right now, or its a voucher for special games.

right now the 3rd post is a voucher, but its only working for watchdogs or ff3

There was a "hidden" code in the goat infographic but I don't know if it has expired http://blog.playfire.com/2014/05/green-men-who-stare-at-goats.html

Whenever I see a coupon section during checkout I simply google it and 9 outta 10 times I end up finding a coupon code that actually works, there's a couple sites out there dedicated to collecting coupon codes for sites like the one you mentioned.

Sign up for the email. You will almost always get the code in the email if it exists.

Theres one under that my little pony toy on the end of your bed.

UK3A0U-YOVK88-88EMC8 coupon 20%
for elder scrolls online