I just ordered some used tennis balls off Ebay but have not recieved them yet. I worry they will be all dog chewed or dirty...

But I could not afford new tennis balls and even on Really Good Stuff site they are 16 for six chairs worth.
I think my bid, with shipping, totalled $35 and I will have enough to seat a class of 20. My husband will take a box cutter and make x's in them to slip them on. The last time I did it, I made the cuts too big and the balls would come off the bottom of the chairs eventually.
Walmart used to sell brightly colored tennis balls with smiley faces on them and I used them for years. Once I even found them $2 for 12 balls! I bought everything they had. I found that without the squeaking and scratching of metal on tile, my entire classroom was more pleasant to be in. The children seemed calmer and happier. This year I am expecting a very wiggly and immature bunch so I thought I'd pay whatever price I could get...sigh. My brother in law is trying to convince his tennis club to save their balls for me. I hope none of the members are teachers who want them for their own.