I see this post was from 2011, i am doing the same thing to my regulators and it was hard to find the part. I ended up finiding it at micromatic.com . it was the most reasonable. still cost me almost $12 after shipping.
Anyone tried one of these : http://www.northernbrewer.com/shop/draft-tower-extension-1-to-3-faucet-conversion.html ? Ive heard a few people talk a little about them but have not seen where someone actually used or has seen one. For $60 I think its worth it.