Tahoe chips offers therm but they are WAY too much.I bought 250 from JP'S CORNER and got foam rings with them and it worked out to about 35 cents each if Im remember and they gave me excellent service.
Here is the link:http://www.jpscorner.com/
Give them a look.
Tahoe sells the rings separate I think and I know that he actually charges for the circles inside of the rings as well.I have heard of squeezing a buck...but that is pushing it.