This maybe one of the easiest purchases,
www.mcmaster.com - these guys ship faster than anyone else around, item number 6663K21 looks like what you want, its a 19 tooth with a 3/8" bore (its on page 929 of the catelog)
www.bostongear.com - high quality stuff
www.martinsprocket.com - also great stuff
www.sdp-si.com - also great
have fun, all of these guys ship really quickly with great products at great prices.