emmm dont unlock the phone cause tht voids the manufacturers warrenty and they are renouned for going wrong, however if you still insist on getting one then you might have to go to contract or buy one sim free and put your current orange sim in it. Although buying them sim fre you are talking around £500.
If you want orange specificaly then try an orange phone shop as far as i know orange do them and there contracts at the min are very good value