i think your best bet is a couple of places. maybe some NTers can chime in if they don't want to give up their method.
a. I usually scour ebay pretty frequently. But I miss out on stuff that's been listed at a low price. I think I need to set up ebay notifications or something. I'd recommend doing that too
b. instagram.com/sellkicks - there's a lot of people posting on here. some legit, some not. just gotta be on point, but they have some gems and pretty cheap deals from people
outside of that, places like SC markets, but I havent had a transactions through them in at least a year.
all in all, i do pretty well with what I want via eBay