Best practices for sourcing wearable activity trackers from Alibaba?

Perhaps you could start by searching AliExpress?

As Jason has said try aliexpress for a small order and take it from there.You may find the main supplier and go direct 

Hi Betty, Shape Up RI (Rhode Island) is a wellness organization in Rhode Island that provides that type of fitness tracker to participants.  Their website is 'shapeupri.org'.  They would be able to help you source this type of fitness tracker.

Software and the filtering parameters are the key to making these devices work well.  Its less the device, more the software you put behind it

Writing this from Hong Kong.

What you want to do is to find a few shops at random and order from them a standard product with minimal customization. It turns out that most of the manufacturers end up shipping with reasonable quality, and the price turns out to be rather standard. Once you have a supplier that you have a good relationship with you can customize things more and more.

A few tips

1) do not buy from a supplier that offers unusually low prices. For most goods, the price quotes are pretty close to each other
2) Related to 1), don't try to drive down the price too much. The manufacturers are operating at bare margins, and if someone agrees to drop the price significantly, this is usually a bad sign,
3) usually the hardware is good, but the big area where there are QA problems is software and things like documentation
4) if you require anything that needs a custom product, then you likely want to go through a development company (like Global Sourcing, Berkeley Sourcing, or Radius Development)
5) always have two or three suppliers

One other thing is that my company provides trade financing.  What happens  is that for small orders (USD 10k) I pay the factory upfront, and you can pay me back in installments.  This gives me an incentive to make sure that everything works, so before I issue the loan, I'll make sure that the factory looks reasonable.