I use a Samson wireless mic. It’s fantastic!!! I don’t know where you are but I got mine through Axe Music. Here is a link. If you’re in the States I’m sure you can find one there!
I agree with Donna. Samson is really nice. The important part is if it the headset is compatible with the stereo. I had problems in the past.
Check with other instructors in your area what they use.
A helpful link could also be: http://www.pcmag.com/
they review all kinds of products and I found some helpful insight with them. Good luck and save your voice. Great articles on how to project in the idea library.
I use the Samson wireless mic too. Great mic but it can be fragile and it uses a lot of batteries. Be sure to use the sweat guard. You need to have a receiver connected to the stereo and the headset. Make sure the same channel is not being used by another stereo in the building (for multiple studio gyms).
I also use Samson wireless mic. If you are teaching cycling, use the backpack version as the sweat will eat up the wireless unit. If you get the wireless, with the transmitter in the headset, make sure you order the sweat guards that go over the little unit on the headset. Wipe down your headset after each use too.
I have used Samson for many years. You can contact KB Electronics in Madison Heights VA. One reason is they do all the repairs from Supreme Audio and others. They sell the units. I have dealt with them for over 10 years. 434-386-6262.