OP - these deaths are most likely happening because of the issues raised in your other thread
on this matter.
But about sourcing.
For me, I pretty much refuse to buy from any large-ish retailer or any of the web-based companies that claim to be the largest in the country/world. They're frequently straight-up lying (largest? haha, no, not even in the US), which befuddles me, and most of them have been keeping shrimp for such a short amount of time that they don't fully understand what they're doing. When you're retailing a bunch of imports on a large scale, it's just not possible to provide the level of care and observation a typical hobbyist provides. Some of them claim to do 30-day quarantines but... I'll bite my tongue so as not to alienate everyone. Just do a lot of research before buying and don't take risks with unknowns if you can help it.
My only exception to the importer rule is Rachel O'Leary because she's a serious hobbyist.
Large: avoid if you can. Smaller = usually better. Experienced hobbyists on forums like this = best. Feedback ratings for members = a good place to start. Not everyone will have tons of transactions but most reputable members with quality livestock will have enough feedback for you to get a sense for what's up. Most will also have tank journals that allow you to see how their critters are raised.