While on holiday in the Isle of Wight I discovered a wonderful shop called Upstairs Downstairs. It is located close to the pier in Sandown.
While it is principally a model train shop (and some dolls house stuff for the ladies) it stocks a huge range of scenery supplies, such as flocks, trees, flowers, hedges, walls, water features, etc. The staff are very helpful and know their stock inside out. The lady who helped me made some brilliant suggestions and even found cheaper solutions to what I needed that better suited my wallet. They also stock a wide range of train set buildings, many of which can be used for wargaming too if your budget stretches that far. They even sell the illusive N gauge buildings, which is good for 10mm-15mm scale wargaming figures.
They also have a great website and orders over £30 have free postage in UK (orders under £30 have a flat £3 P+P charge). Click on this logo or here for the link.