I have not seen resistance bands in Hanoi, but the people at Zenith Yoga might be able to help you, as they teach Pilates classes at their studio as well as yoga. Marzena is the main pilates teacher there. You can e-mail her at zenithyoga@yahoo.com or stop by the studio at 111 Xuan Dieu (on the 4th floor). Good luck! If you can't get any here, I'm sure they can be picked up the next time you're in Bangkok, if that's an option.