If you're looking for ready-made seamless textures, the following URLs will direct you to some available resources. Most of these sites include free samples for non-commercial use.
Absolute Textures
http://www.absolutetextures.com
arroway.de
http://www.arrooway.de/en/index.html
Dosch Design
http://www.doschdesign.com
Got3d
http://www.got3d.com
Marlin Studios
http://www.marlinstudios.com
Realworld Imagery
http://www.realworldimagery.com
Sub Dimension Studios
http://www.subdimensionstudios.com
Texturama.com
http://www.texturama.com
The3dStudio.com
http://www.the3dstudio.com
The Digital Artshop
http://www.thedigitalartshop.com
TiledTextures.com
http://www.tiledtextures.com
Turbo Squid
http://www.turbosquid.com
Ultimate 3D Links
http://www.3dlinks.com
MFM
Absolute Textures
http://www.absolutetextures.com
arroway.de
http://www.arrooway.de/en/index.html
Dosch Design
http://www.doschdesign.com
Got3d
http://www.got3d.com
Marlin Studios
http://www.marlinstudios.com
Realworld Imagery
http://www.realworldimagery.com
Sub Dimension Studios
http://www.subdimensionstudios.com
Texturama.com
http://www.texturama.com
The3dStudio.com
http://www.the3dstudio.com
The Digital Artshop
http://www.thedigitalartshop.com
TiledTextures.com
http://www.tiledtextures.com
Turbo Squid
http://www.turbosquid.com
Ultimate 3D Links
http://www.3dlinks.com
MFM