HTC Desire Z SIM unlock code?

When I insert my Rogers SIM card into this HTC phone (originally from Bell), it says I need a SIM unlock code. Where can I get this code? Are there any reliable websites that I can get the code from?

(also, does this code ever expire?)
Just making it clear, I just got this phone. The problem is the phone is blocking my Rogers SIM card.

I suggest you can get a permanent unlock code from Unlocking4u.com and remove the network lock from bell network in your HTC Desire Z phone.They also provide free instructions to aid in the process.