How to find out who designed a site?

Does anyone know who designed retailmenot or befrugal

Anyone capable of doing something similar

Did you try checking the source code?

Yes but I'm not really sure where to look, I don't see it in the obvious place it would think towards the top or bottom

It's common to have some sort of author info in the comments inside the code. That would be a good place to look, the whois data might be able to find it too, or at least it will give you some sort of contact with the company if they haven't made it private. If that does not work try archive.org look at an old version of the site often this has the contact info right on it.

A coupon site eh.

Why would you bother looking for the developer? If it could be installed or built without the updating of the coupons with expiry dates and stuffs, it would mean that it is "for show" and may not be that legit. Or in simpler sense, the coupons won't work.

Building something like that through legit means would definitely take up a lot of time and networking skills. You would require connections with the websites which are offering the coupons.

Or if validity isn't that necessary you may harvest the coupons through the search engines with proper keywords which is what I think if you have found the developer, they will provide you with this.