Yea, the buttons aren't made anymore - so you have to find them on newsgroups or ebay. Notice that the alternate solution mentioned in the link above does not include the LED or lens.
One way to find volcano buttons is to look for complete control panels on ebay that happen to have the buttons, then ask the seller if they would be willing to drop some of the shipping price by just sending the buttons without the panel.
I got lucky by finding some that were listed on ebay as 'arcade buttons', so a lot of people just passed them by without checking out the picture.