3,25x4,25" filmholders are difficult or very expensive to get here in Europe. (Got 2 from KEH with the help of this group, but that means only 4 sheets at one load.)
9x12cm casettes are easy to come by.
Do by any chance would they fit the back (springback) of my 3,25"x4,25" Speedgraphic? I know the film dimension is different, but would the holder fit?

No. 9x12 (cm) holder will fit 4x5 (inches) backs, not 3.25" X 4.25". " means inch.