Best advice I can give you is to forget ordering online.
Pop down to your local industrial estate and hunt down plastics suppliers there - most will have off cut bins with anything up to 2 foot sheets of 'scrap' of various colours... find the foreman and haggle with him.
Of course, you can also buy full sheets from them.
cheers guys well i took your advice and after a trip down the local insustrial estate found a sign manufacturer who was happy to cut to size the piece i needed for only £6