Bitmain.Com, manufacturer direct, as reliable. Purchased from since 2017 when reasonably priced stock is there.
   note : Their customer service (email response) has been horrible the last year. They used to be prompt and answer the actual question.

I can also strongly recommend Whatsminer.net (not Whatsminer.com). I have purchased from them in the past (new unit about 2 years ago) and just recently a Certified Used M30S that arrived within a week of purchase. That was unexpectedly fast. They are lacking in new unit availability (chip supply shortage), but keep listing batches of certified used. They are a US based factory re-seller. I THINK  only 1 of 2 official US suppliers.

I have also used Amazon, but expect to overpay. (Purchased from Unistar on Amazon in the recent past)


There is a number of threads with some reviews of sellers (mostly on Telegram) if you search the forum. I have one pending order I can't speak to reliability yet, but has been reviewed as a trusted supplier by other members, is Grace Zhou from minerexpert. Again, I can not speak to reliability myself yet, but have 1 unit on a future batch I have trusted sending my money to based on other forum members recommendations.