I’ve always got them from eBay.

The T and X series are the high-end ones. Between those it mostly depends on what size of laptop you’re looking for. Its worth checking a guide for how you replace the SSD/RAM/battery - some of the newer ones have these soldered in place, which means you’re stuck with whatever it originally came with.

Personally, I think the sweet spot is around 4 years old. By that point they’re pretty cheap (maybe 10% of the original RRP), and going for older ones doesn’t save you much more money. I recently got an X390 and it’s doing everything I need from a laptop