Microsoft lets you order the newest service pack to be sent to you on a CD for $10 here
, or they do have the full download here
you will have to download the Network Installation version (other version requires an internet connection on the comp you are installing to), which is 129MB, and burn it onto a CD, or other portable media that can hold 129MB (USB Thumb Drive, Zip drive ect.)
*edit* heh, I took too long to type my post
MasterJason beat me to the full download link.