There are some regional players in other areas (SaskTel, I think in Saskatchewan for example).
The best rates for pay as you go are from Virgin, however their phones aren’t subsidized so they can be expensive (they have a $50 model though I think), and their service won’t work in the US (they don’t a have a US roaming partner yet).
The other providers have great deals on handsets, but usually only for contract terms (1-3 years).
You could always look into renting one for your stay. (see here, and here). Google for other rental places.