Ken over at The Maker’s Box has used the website circuits.io to design a board with 8 channels of analog input with the ability to add another board on to give 16 channels! They’re not for sale – he just wanted to create something to use with his model railroad. Read more here
Where can I find this IO board already made?
As it says in the blog post, the maker is not offering them for sale. However, if analog input is what you’re looking for you might want to take a look at the Explorer HAT Pro (http://thepihut.com/products/explorer-hat-pro) or alternatively look for resources online using the MCP3008 chip with the Raspberry Pi. I’ve used the chip a lot before and it’s a pretty simple way of getting 8 analog inputs.