you can look in yellow page for this.but be carfull don't deal with too small comapny, as that company will be responsible for your serice later on, just like enbridge now.too small comapny , may not have good service, eg, furnace break down in the middle of winter night ..
to start with, first find the right brand you like( lennox.carrier,kenmore,armstrong.... ) , some are more expensive than others.
Then find the dealer who sell those brand. normally one dealer only sell one or two brand.
second , after deciding the brand check their website to find out which mode ( or ask sales people)
third, call them to find out price.
I am not too sure about switching from electric to gas, it may cost extra money.
My experince, chinese dealer generally cheaper, but not too good at service. I find out the price from chinses dealer and force canadian dealer to match price.
I choose lennox, by the way. Good luck!