Try Craigslist. I don't know if many people would have them listed now, but surely when the weather starts to get nice. Also, keep your eyes out for a Toys R Us or Babies R Us 20% off coupon. That'd be a big chunk off of an expensive car. One word of advice....be careful if buying one used. Make SURE the battery is good or it comes w/the battery or charger. If it doesn't work, the battery alone could cost your $50 or more.
I see that your son is 15 months....a car-car might be a little difficult for him at this age. My son was 2 1/2 when he got his Lightning McQueen car and that was in the winter at Christmas. So he was close to 3 before he really started driving it. He's a PRO now though.
They have a Fisher Price Kawasaki "bike/motorcycle" on sale at TRU right now. Regularly $100 for $60 and it got good reviews. My son is turning 2 this week and that's what we're getting for him. It's also battery operated, but they just have to press a button instead of actually having to push on the "gas" Just a thought.