Tractor Supply Company. However, I haven't seen it there is several years. Look online and you should be able to run it down. If not, Harbor Freight has a "Brake Bleeder Adaptor & Tubing," #98864, $3.99 that should work:
http://www.harborfreight.com/cpi/cta...emnumber=98864
I haven't tried it, personally, though.