You can buy the codes on $bay for £18 - £25. Other than ringing three and asking them for the code there is simply no other way at this moment in time (unless you buy an X-sim and cut your sim card)
I know trading isn't allowed, but as a one off I can supply the unlock code for £15 (this is wholesale with paypal chatges included. Don't really think this can be classed as "trading"...I'll even give you the unlock code 1st to make sure you are happy before you pay.