Perhaps have a word with the local Council and see if they have any old ones knocking about. They might pass you on to Highways England depending on who's remit this falls under.

It might also be worth contacting a few salvage yards - this one has a lot of street furniture and that includes pedestrian wait push buttons for £35, part of their pedestrian crossing parts section. Although as scrap yards list on eBay these days - pedestrian crossing boxes go for about £60 with one or two up for bids at lower prices.