Many of the buysell ads are for unlock codes which are obtained by sending your phone's IMEI number and a fee.
For your money, they send you an unlocking code and instructions. You put a SIM card from a competing network in your phone, and when you re-power it, it can't sign onto the network, so the instruction "enter subsidy code" comes up on the display. You then enter the unlock code, and it should unlock your phone and sign onto the network for the SIM card.
I followed all the instructions, and the code I bought didn't work. The website on which I bought the code promises "100% satisfaction guaranteed" but still hasn't refunded my money. They want to refund part of the money, since they paid for the code and think I should cover their costs. Apparently, getting a 100% refund for a guaranteed service that doesn't do as promised is more than 100% satisfaction.
I'm taking it up with PayPal.
Anyway, I went to one of the places recommended by ckl, where they unlocked my phone in 5 minutes (using an instrument rather than a code) for $15. My phone worked on FIDO with a FIDO SIM card, and it works on the original network, Rogers, using my Rogers SIM card. Success.