If you know who supplied your windows you may be able to get new keys - they are probably all common-keyed.
Oddly enough there is a wonderful tool called Google that allows one to search the Internet for all sorts of things. Why not give that a try?
I searched for "double glazing keys" and got this as the very first result:
http://www.windowhandlekeys.co.uk/
Anon
July 2016