Car Boot Sales are a great place to find retro hardware from games and consoles to classic computers and peripherals.
The first thing you will need is to the find the biggest regular boot sales in your area, you may have to travel to find a really good one!
I use this site www.carbootsaledirectory.co.uk
to keep me updated on sales near me, and find any new ones that have started, a good site as it has maps and contact details for most of the regular boot sales I visit, and it can be used from your smartphone!
An Amiga 500 , ok so it came with no box or power supply but purchased for just £5 last year!, and it still works.