Hello - thanks for help here and on the bad back one! Having trouble, found a place yesterday, but they wanted to send it away and charge $40. Doesn't seem worth the hassle, although annoyed I bought the pre-pay package before I knew any of this!
Ok, if you google the model number, you might find help.
Otherwise, I got mine unlocked (also a Sony Ericsson, strangely) last summer at a stall in the Dufferin Grove Mall. If you are home with non-baby DCs, the Dufferin Grove Park, next door, is, I swear, the best playground in the universe. Just next to Dufferin West (I think that's the name?) subway.