not sure if apple will sell you panther now or not. if it's DVD based then yes you need a combo drive to install it. or another computer with a combo drive to install it. i heard you can use target mode or what have you to use drives off other machines over firewire (still haven't tried this, wish i had 2 macs

).
if apple does still sell panther, you could probably get the cdrom version from them for an extra fee of like $10 or so.
if you're talking about Tiger (10.4, that was just announced) then goto
www.apple.com, click OS X on the top bar, then scroll down on the right side there's a box for Media Exchange (i think) that for $10 will allow you to order the cdrom version