Car ownership is mostly a series of small, annoying costs that eventually lead to a big one. Tires are that big one. They are the only thing touching the road, and they usually wear out right when you do not have a spare grand sitting around. You will find the softest prices in October or April when shops clear out old stock.
Negotiate the price of the rubber separately from how you plan to pay for it. Ask for an out-the-door price that includes mounting, balancing, and taxes so there are no surprises. Once you have that number, we can look at a way to spread the cost over a year and a half.
This is a deadline, not free money. If you still have a balance when the window closes, the standard interest rate will apply to whatever is left.