Floppy drives are pretty cheap - I think I paid $12 for a Sony from Newegg . However I'd check Microsoft's website to see if Vista has the RAID drivers for your mobo built-in. For example, with XP original (no SP) I had to either stick in a floppy with the RAID drivers during setup or create a bootable CD with floppy emulation (RAM disc) and copy the RAID drivers there, then run Setup from the DOS prompt. However with XP Media Center Edition the RAID drivers were included on the setup disc. Since Vista is even more recent it's worth a shot to check and see, maybe save $12 . Or just borrow the floppy drive from your old computer to use temporarily.
It depends on your chipset. I was able to install Vista to a RAID0 array on my nForce 570 chipset without a floppy drive or driver, it detected the array automatically. XP SP2 did require one, though.
If it does require a driver, just slipstream the driver for your RAID controller onto a new Vista installation DVD using vLite or something. The wizard will let you slip extra drivers onto the DVD image, which you then burn to a blank disc. Then you use that to install Vista on the array. But you don't need to slipstream the driver if Vista already supports your RAID controller.
What chipset will you be using, by the way? We can help you more if we know your system configuration.