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