You will not find any patterns preinstalled in ~./gimp/2.8/patterns that folder is for your patterns.
The ones you see are the .pat files that come with Gimp, look in
/usr/share/gimp/2.0/patternsThey do not have to be in .pat format, you can use png or jpeg images as patterns.
Background (no pun intended) on Gimp resources here:
http://libregraphicsworld.org/articles. ... icle_id=30Pattern images can be referred to as tiles, and and you might see a join between the tiles, intentional or not. Others are referred to as seamless, where a small tile will fill a large area. The standard Gimp patterns are small 'seamless' tiles.
Some more examples of this here:
http://www.texturemate.com/category/ima ... es/digital and if you look down the left side there is a section
seamless.
The others on that site are described as textures. These do not tile seamlessly but are usually large enough to fill a reasonable size canvas.
Some more locations, mainly textures,
http://mayang.com/textures/http://textures.forrest.cz/and some nice patterns for borders.
http://www.craftsmanspace.com/free-patt ... terns.html these are vectors (svg) that can be imported into Gimp.
Otherwise trawl through the forum resources section also search through
http://www.deviantart.com/