['97ventureowner]

Welcome to AF!
The tires can be purchased at www.tirerack.com or www.bigo.com . You could also call around some of your local tire dealers and inquire if they carry them or can get them. I wouldn't go to the dealer as you will pay a lot more for them, that's if they can even get them. Most of the time when GM manufacturers cars they put out a contract for a certain # of tires they need to fill an order. Tire manufacturers then bid on the contracts in hopes of getting the contract. Many times a different tire or even manufacturer can be used on the same model during the production cycle.
Concerning your first question. I'm not sure exactly what you are asking. Are you trying to find out how much it would cost per month to buy a 1981, 86 or 87 Cutlass 8 cylinder car? If that is what you are looking for , the answer depends on a number of factors. First and foremost the financing costs if you take out a loan to purchase it. The rate and monthly payments can vary based on your credit score and where you get the loan. Also how much the vehicle is worth, and it's selling price. Theoretically, you could pay as little as under $100/month up to over $250/month, once again dependent on a number of factors. I hope this answers your questions. If not post again and we'll try to help.