Wayne,
I googled SI and came up with this........
http://www.thinkgeek.com/electronics/audio/6bd8/?cpg=wnrdfI am sure there are others. Keep in mind fellas.....you CAN liberate the exciter from the cardboard circle. And the description on this site seems fair.......quality can be from OK to spectacular depending on what material and how much you use. I have gotten results equal to the best ribbons I have heard......But I have no idea of the lifespan of the exciter......However they do have a polyswitch built in and will shut off if driven too hard.....easy to do with the Pass X 150! They come right back on after cooling off. Fun stuff.
Ed