Nico you could try and get a hold of a mold-tech book and scan in some textures for bumps. They are one of the big suppliers of molding textures for plastics.
http://www.mold-tech.com/ Not sure if they have samples on their website, but if you dig around a bit maybe there would be something you could download directly from them. Course it is probably too low res.
We have a catalog in the office. If I have some free time this morning I will see if I can scan one in for you and you can make it tileable.
Industrial Designer
Concept Center International
www.ericlagman.com