Driving on bald rubber when the first freeze hits is a gamble you don't need to take. A solid set of winter snow tires keeps you stuck to the road when everyone else is sliding into ditches, but replacing them always seems to sneak up on your bank account.
Prices are softest right now in early autumn before the November panic sets in shops. Knock out the purchase before the demand spikes, and negotiate the installation and balancing into the upfront quote before you pull out any plastic.
Remember that the 0% intro window is a hard deadline, not free money. If you still carry a balance when those 18 months run out, standard interest rates will kick in on whatever is left.