They actually make case mods to fit regular DVD's
http://www.xbox-modchips.com/viper-gc-case.htm
If your copying the Disk how the info is read makes no difference as long as the disk is an exact copy. Windows can not read Xbox format disks but I can burn them easily and they all play fine.
As for the Disks I am Canadian and can find them all over at places like Staples, Future shop etc. I am not sure of the cost but you can buy both the CD minis as well as DVD minis.
I don't know much about Gamecube mods though so who ever I get to install it (If i do) will hopefully know more I hope.