Hi Cutter
If you want the pic printed at A1 and then fit a mount over with a A1 opeing to see your pic then as the others said you have to get a custom made one
Picture lizard are quite good and not bad priced
Or have you thought of print the picture at A1 with a 2" to 3" boarder on the print yes the picture will be slighlty smaller print still A1 , but makes it easier to get a standard frame and mount