Seconded for AliExpress and OrangePi, though it seems like certain OrangePi models are really hard to find or no longer in production.
For routers, it depends on what you need exactly. Getting old refurbished Wireless-N routers on eBay or Newegg or Amazon might be the cheapest option.
I recently tried to DIY a cheap router solution based on an SBC running OpenWRT, but once you tally up the additional costs like an SD card and USB wi-fi adapter, it comes out to about the same cost as buying some cheap/used off-the-shelf device. YMMV.