TeX is freely redistributable. You can obtain TeX for Unix systems via anonymous ftp or on physical media. The core material consists of the Web2c TeX distribution (http://tug.org/web2c).
Instructions for retrieval by anonymous ftp and information on other available distributions:
ftp://tug.org/tex/unixtex.ftp
http://tug.org/unixtex.ftp
The Free Software Foundation provides a core distribution on its Source Code CD-ROM suitable for printing Texinfo manuals. To order it, contact:
Free Software Foundation, Inc.
59 Temple Place Suite 330
Boston, MA 02111-1307
USA
Telephone: +1-617-542-5942
Fax: (including Japan) +1-617-542-2652
Free Dial Fax (in Japan):
0031-13-2473 (KDD)
0066-3382-0158 (IDC)
Electronic mail: gnu@gnu.org
Many other TeX distributions are available; see http://tug.org/.