I recently took a risk on this one. It has good reviews, although they could be fake as there are only 5.

https://smile.amazon.com/gp/aw/d/B01MA5930J

I've only used it once but I didn't notice a problem with the magnet.

I made the mistake of purchasing one from Ebay it works but the magnet is weak and you have to play with it. I purchased another from Amazon and its as good as the original one that came with the watch! This one looks good, Amazon reviewers check out ok: https://www.amazon.com/Zenwatch-E-fashion-Portable-Replacement-Zenwatch2/dp/B01MA5930J/ref=sr_1_2?ie=UTF8&qid=1502632859&sr=8-2&keywords=asus+zenwatch+2+charging+cable