Yes. You can buy PlusBus day tickets (at the same time you buy a train ticket) at self-service ticket machines at stations managed by the following train companies:-
Avanti West Coast, Greater Anglia, Great Western Railway, East Midlands Railway, South Western Railway, Southern, Southeastern.
Self-service ticket machines at main stations managed by LNER and First TransPennine Express also sell PlusBus day tickets.
To see how to serve yourself with a PlusBus day ticket, click here.