Look in the For Sale section. There is a guy selling them there for ~$5 a set I think.
However, there is a less expensive alternative. If you can put up with a little advertising, Graco Corp. will send you a bunch for free. I don't use them so, I gave all mine away. If you want some, just give Graco a call. If you don't like the Graco name on the side you can turn them the other way around.
LM Lenses has some listed under the shooting glasses-->accessories tab on their website at $1.00. Doesn't say if that is for a pair or each. If it is for a pair they are cheaper than ones on ebay. Hit the link above.
heck, just take a couple 4-inch-long pieces of duct tape and fold them in half to make 2, 2-inch double-thick pieces. Use a thin strip of tape to tape one to each ear piece. Chuck them when you're done.