No no no...linux itself is free. You CAN buy commercial CDs, if you want. But you do not have to. You can download RedHat 7.3 ISO images from
ftp://ftp.redhat.com/pub/redhat/linux/7.3/en/iso/i386/
If that happens to be busy, any site on this page should have it:
http://www.redhat.com/download/mirror.html