Hi. I too had this problem and spent hours searching the internet for a supplier. I eventually found this website www.hdhardware.co.uk which supplies every type of gasket you could imagine (and their prices are reasonable). Hope this helps.
simmo1992
November 2010
Hi
How much gasket are you needing as it would cost about £40 for 50metres for this gasket.
craig
November 2010
This is a black rubber seal that is on the inside of the window that you force between the glass and the frame to make the windows stable and an air tight seal. The windows are about 15 years old and are not made like this anymore.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix windows and doors.