I've got the Polar M400 and it's been great so far. One or two hiccups but that's technology for you. I picked mine up in New York but as everyone else is saying, eBay is a good bet.
As Brian suggested, Polar Beat is great for going out for runs and is more than likely to be cheaper than buying a dedicated watch but if you are getting a HRM for use in matches too, you may not want to take your phone on the pitch with you