The size you've mentioned is a non standard door size and as you're presumably aware you can't shave much off a fire door before it no longer complies, there are door blanks available, two suppliers that spring to mind are http://www.edens.co.uk/default.aspx?pid=122
or Howdens http://www.howdens.com/pdf/product/454.pdf although you'll need a trade account with them or buy via a tradesman. There are probably more suppliers though if you search.
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