Best place I know for thinnish coloured plastic is Fred Aldous of Manchester. I bought a yellow and red A1 sheet for around a fiver each last year and delivery was excellent. Give it a go although you may not get the exact match but I was well pleased and have enough over to replace another backflash should another restoration project come my way.
http://www.fredaldous.co.uk/
http://www.fredaldous.co.uk/
