Amazon currently has the below on sale for $7.07.
Sterling Seal ORBN008x1000 Number-008 Standard O-Ring, Buna Nitrile Rubber, 70 Durometer Hardness, 3/16" ID, 5/16" OD (Pack of 1000)
The price has been dropping a couple cents every few hours. This is great deal for o-rings that quiet down mechanical keys that most people bottom out on. Standard keyboards take about 104x rings and add few extra for they larger keys like space bar which requires 3x rings. This should cover a couple keyboards and you can even try double up on rings per key just to try out.
The average price for 104x from a quick ebay search is $1 - $9. So this deal seems great to me and no waiting for the cheap stuff shipped from china.