From previous experience at forest, they sell blocks of tickets to home supporters, if there are unsold away blocls, they ask for them back from the away team's ticket office, so that they can be sold to home fans. They also keep an additional area for pay on the gate away fans.
However, it is unlikely that there will be a full house, so hopefully there will be some available on the gate, but it would be worth calling them to see how many. Best bet is to get them from our ticket office, before the weekend