Since it's `Frage nach` one would think that it would be `Anfrage nach`. I can't tell you if it's completely incorrect, since historically, it wouldn't have been used this way in the first place. (It would have been something like `anfragen bei jemandem + relative clause`)

Still, I wouldn't recommend *nach*, because (following my intuition) the two prepositions, *an* and *nach*, don't quite fit together.

*Anfragen* in connection with some noun usually comes with [*wegen*](http://german.stackexchange.com/questions/15093/use-of-dative-after-wegen/15106#15106):

>Wegen dem Angebot anfragen  
>Wegen des Angebots anfragen

Note however, that this is not asking for some offer, but about it. Also consider simply using *Anfrage*, if it's clear from context what the *Anfrage* is about.