finding an agent for a screenplay?

[aftereight]

Hi, this is probably an age old question, but DH is wondering how to get representation for a screenplay he's written, when the advice is that agents won't read unsolicited scripts?

He has written a film screenplay and it's been read by a producer in ITV who said in her feedback that she was "incredibly impressed" and said it was "well written and brilliantly plotted" (and various other positive feedback comments). She did go on to say that it would be too big budget for ITV to pursue because it's set in a number of different periods and geographical locations, but that it would be a "great calling card" should he take it to others (non TV).

Does anyone have any advice please on how to get representation?