Here’s a nice cheap deal over at Amazon. Right now they have the Chuckit! Tennis Ball 2-pack on sale for $1.28, plus for a limited time you will automatically get a $0.13 discount at checkout for each one you buy. That makes the final price just $1.15 ($0.57/ball.) Here is the link to get yours:
Chuckit! Tennis Ball 2-pack – only $1.15 ($0.57/ball) with sale and instant savings at checkout!