I got mine on ebay - postage was more expensive than the bar ends themselves but they do add a certain charm the bike i feel - just make sure you get 7/8 (22mm) fittings
they work by expanding rubber but that sounds far more complicated than it is - basically a screw, a washer and a bit of compressed rubber after you've tightened it
these are the ones i got:
http://cgi.ebay.ie/BIKE-MOTORCYCLE-MOTO ... 2c55990b85
hope this helps