My goto for a while was the BT HomeHub 5A (not B variant). Just checked and these are available for £10 on eBay if you're based in the UK. I'm sure there are bargains to be had all over the world. The place to start is to look at OpenWRT's supported devices page and look for ISP branded models. These are often flogged on eBay super cheap when people move because they're basically scrap to most people. I'm sure even more just go in the bin (sadly).