Depends a bit on your definition of vintage, but roadside finds are far from vintage IMHO. The oldest you're likely to find these days is Pentium 4 / Athlon XP.
We used to have a great thrift shop here in Amsterdam with a dedicated not-for-profit computer section that offered computer courses to the technically illiterate and rebuilt stuff they got donated to working systems running FOSS which were sold for low prices. They knew their stuff, had great stock of pretty much everything from mid 1990s onward for decent prices and were always willing to help if you were looking for something specific, particularly if you reciprocated (my employer frequently dumps old but still perfectly usable network hardware, which the thrift shop guys loved). Unfortunately they were too non-profit and ideologically motivated for their own good and went bankrupt last year. Since then I've been looking for an alternative, but haven't found it yet, and even if one appears, it will start from scratch i.e. the stuff people are dumping now, which is too new for what most consider 'real' vintage.
Two years ago I participated in a large online community-driven effort to donate hardware and build systems for charity. We received several hundred systems. Over half came from companies and were typical written-off office small-form-factor desktops, but the other half were private donations spanning everything from high-end gaming rigs to shop-bought bargain basement systems. It was great fun, I saw more diverse hardware than I had for a very long time (and put some once obscenely powerful high-end components into otherwise low-end systems 😉 ), but it was telling that we only received a handful of beige cases, and the oldest we saw was a late-model Pentium 3.
Basically that means I'm now down to garage finds and dead mans shoes - systems that have either been forgotten for ages, or have been used (or at least owned) by pensioners until they passed away and the next of kin need to get rid of the stuff. Local Craigslist-equivalents (here in NL: marktplaats.nl ) seem to be the best place to find them.
Or just luck on eBay. Sometimes people sell old crap for low prices, or misspell names or model nrs so that most bidders miss an auction.