You can buy them on xbox website for $12 and change, sadly it takes 2-3 weeks delivery. You must speak witha custserv as the transfer kit is no longer a selectable accessory on their site. Item will be delivered by regular US postal service.
...Or
Go to a large dept store like Target or Walmart and buy a 120GB HDD, take it home, open it, use the transfer kit, then bring it back for a refund. I suggest dept store since their return desk is less likely to hassle you for something like this since they get returns constantly.
xbox custserv says there's nothing they (want) can do to help me other than sell me a transfer kit.
good luck