I have been buying 12" x 10" frames from Sainsbury's.About £7
These have a 10 x 8 mount included which i have found stops the autograph from touching the glass direct.
They do these in different effects(Black,Pine,Etc.)
Also,Wilkinsons do 12" x 10" frames without mounts in some lovely finishes(Chrome,Etc).
They sell 10 x 8 mounts seperately.