Previously I have had good luck on r/homelabsales but it seems that there is not something similar on Lemmy yet. eBay has worked too but takes much more effort to find a trustworthy seller.

Lenovo and Dell have outlet stores online but the savings are not as high as a couple year old device would have but at least you have a manufacturer warranty.

I know someone who purchased several refurbished units for his start-up from Amazon and worked out well, I would just make sure to check return policy before trying Amazon.