I have only used 2 kinds, and would recommend either one:
Polar H7, paired with Polar Beat app and Polar Flow web site (also pairs with Elite HRV app). The strap is flexible and works well no matter what you're wearing. That said, I have 2 of the H7 devices, and the second one works a lot more reliably than the first. Maybe they are inconsistent in how they are built? The first one just doesn't pick up as well, have to be more diligent about putting water or gel on it, and careful placement, even when the battery is fresh. The second one works well no matter what I do. They are the same model, just bought them at different times. Cost is about $50.
Garmin, mine is an 8-year old model that I use on my bike, Edge 705. The strap is about 12" long, flexible, and fits in the sports bra without even using the rest of the strap. The Garmin receiver only talks to Garmin devices, and I think it's RF rather than bluetooth. Cost depends on the device you buy.
I would recommend getting one (probably the Polar) -- very much worth it to see where you are HR-wise with your sessions. Even when you're not that concerned about HR, it serves as a handy recording device to let you know what you did on a given day, which helps if you forget to log it or want to look back at history.