Although the 2 places Alpenwasser mentioned, Lutro0 and Pexon, Are great they are Very Very expensive.
I got all my 24 Pin Power, 8 Pin EPS, 6 Pin PCI-E and 8 Pin PCI-E cable  extensions from cables4pc, He/she is based in Surrey and his/her work is  amazing and IMHO is just as good as the ones Alpenwasser mentioned but  don't require you to chop off your nut sack, Scoop out a kidney, Cough  up a lung and sell it all on the black market to be able to afford them.
I've  got a friend who bought the Premium PC Extension Kit and to my eyes and  fingers I really couldn't tell the difference between Cables4pc's work  and Pexon's work, Cables4pc for an entire set i.e 
24 Pin Power, 4/8 Pin EPS, 2 x 6 Pin PCI-E, 2 x 8 Pin PCI-E  costs £40 and the exact same thing from Pexon costs £70 and in all  truthfulness I could not see or feel where the extra £30 went but I'm  guessing it's because Pexon does all of the work himself.
http://www.cables4pc.co.uk/
The other cables i.e fan cable extensions, Front panel extensions and  SATA extensions I got all from Bitfenix, Their cables are awesome, They  look professional, Feel soft and cotton like, Grade "A" stuff and their  prices are amazing as well.
I don't work and I'm not endorsed by C4PC but I've used their products in all my builds and can honestly not recommend them enough, So here are a few pics from their site as an example of their work, These are the one's I currently have in my rig and as well as looking great they have a soft material feel to them 

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*NOTE* You can get any colour combo and cable length you want you just have to send him/her a message 
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Hope this helps 
