Definitely check garage sales. I got a new-old one for like $5 when Sammy had her surgery. I also email blasted people at work asking for a used one and got 2 offers for FREE. They are so big to store that people don't keep them around once their kids outgrow them. Many neighborhoods do a big garage sale with like 10+ houses. Pick the nicest "family" subdivision for the best luck
I've also had luck in our neighborhood (townhomes) because we have no basements and therefore ZERO storage. I know I've personally sold practically brand new stuff for next to nothing just so I could park my car in my garage