Parts of the Meraki OS are open source, and parts are not. You can, for example, request the bootloader used in many Meraki devices and Meraki will provide that to you. I'm not aware of a website though where you can just grab this (it's by request only).
If there is a site that hosts Meraki firmware for you to download it's not hosted by Meraki, and I doubt very much it's sanctioned. It'd also be mostly useless as there's no (official) way for you to load that firmware on a Meraki device as they only take firmware updates from the cloud.