The original sails were almost certainly made by Parker and Kay (now called "1 Sails") at Levington in Suffolk. They would likely be pleased to quote for a new set. They may hold details for the 275 sails but like most sailmakers they would probably want to measure your boat.
There is nothing special about sails for Parker boats so most sailmakers could make you some.
A reasonable option would be to have a talk to a couple of sailmakers local to you about what sort of sailing you do and what sort of sails you want, they would be able to quote you for various grades of sail. Cost can vary widely according to materials used and details of the sail construction.
My personal view is that it is worth paying for the better grades of sailcloth, other people will disagree with this approach.