MY first port of call would be Vodafone. If you are still in a contract with them and have been for more than 90 days you could tell them a bunch of your guys are going overseas for several months (6?) and will need to use other Operator Sim Cards could they kindly unlock them. They may charge around £10 - £15 per handset.
Althernatively someone like
HERE might help you. I am in the UK and i know this company will do them and are very good.
Althernatively, cant Orange give you handsets?