I've bought a few 5m screened cables from here after we went with cheapo jobs at work and found severe ghosting. Much better PQ from these cables. 2129HQ-10 is a 10m Plug-Plug cable. £19.50 (+VAT I presume).
You could also try Lindy. A few of us on the forum are using their "Premium Gold" VGA cables. Top notch stuff, and available in loads of different lengths...including 50m! Outrageous.