On amazon.co.uk - basically a set of wheels to clamp your tripod into turning it into a basic dolly.
£30 including delivery. This will also make the tripod more stable even when it is still. A great price, given how expensive so much camera equipment is.
I also recently bought a hama star 63 tripod which are heavily reduced on amazon. Got mine for £15. Do an amazon search for "hama star" and have a look at what they have. Nicer than my old tripod, a bit heavier, and has a three-way head so I can tilt the camera in every direction. Still a little plasticy but ift feels solid. Cheap but decent!! Gets the job done.