Sorry to add to the various suggestions but found the following Ebay posting for 22 60 cell polystyrene trays used once by a Nursery in Norfolk. See video on the posting and they look strong and reasonably deep.
http://www.ebay.co.uk/itm/Plug-plant-seed-trays-22-x-60-cell-seed-trays-polystyrene-plug-trays-/161882851086?hash=item25b0f8470e:g:zHcAAOSwQoFWQK9B
Hope it helps.
