Used classifieds, but honestly, I wouldn't recommend any* of them for working out with. they're too bulky compared to modern stuff, and the Nano's* are all on the verge of dying.
Aside from the Sandisk there's also stuff like the Fiio M5
and Shanling M0
, they're both tiny and have BT, I'd say pick the one you like best or the one that's available.
*But if you prefer to stick to the Apple stuff, there's one exception to the Nano lineup, the 7th gen. Its tiny like the other Nano's but it has an easily(compared to the others) replaceable battery, has BT and actually comes with enough storage to get a decent amount of music on there.