I've never done it, but you buy it from the thief (junkie) directly. So it could be that you find them by word of mouth, like you would use to buy drugs when they were illegal. But you can also buy a second hand bike very cheaply, some are even free (just an example: https://www.marktplaats.nl/v/fietsen-en-brommers/fietsen-her...). New bikes are usually more than 200 euros, dutch brands are 500 euros and up

Presumably from the bike thieves. There's a glut in the stolen bike market so the price competition is fierce.

Last time I bought a bike for £20 even second-hand, the Euro had not yet been introduced.

Even in 2011, when I made the mistake of spending £90 for a new bike… well, the pedals came off while riding it due to metal fatigue.

Adjusting for inflation*, I'd expect similar build quality from a bike that "only" cost €200 today.

* hard to do when there is also a currency switch, especially when the exchange rate has changed so much

This was 2010-ish and I found one on Marktplaats. Might be going for 40 or 50 now with inflation. People selling them on the street also had for cheap, but these were most definitely stolen.

They didn't mention the year of the story, it could've happened a couple of decades ago.

Police unclaimed bike auctions maybe.