It's sold as different brands. I bought one labeled as ProForm, but I've seen the same HRM as Reebok and Schwinn. Should be about $25. Maybe they're being replaced and the supply is depleted, but I bet there's a few available online. Not a great HRM, but it works. Anyway, you should be able to get an entry level HRM for $50.

The main reason I wanted a separate HRM is that you can use it off the bike. You really need one for spinning class when you really try yo push yourself hard. I think monitoring heart rate is the best way to train.