I very much recommend that you buy 9x12cm film since that is available. AFAIK the 9x12cm film is slightly smaller than 1/4th of a 18x24cm - the "conversion" from glass plates to film makes the real sizes different. I'll try comparing 4 9x12 with a sheet of 18x24 when I get home (I use both sizes).
My first shots with the 18x24cm camera were on cut-down 24x30cm film. I wouldn't recommend cutting down film unless there is absolutely no other option, and certainly not to save money. You have to factor in the cost of mis-cuts!