WISP communities (e.g., r/wisp, WispTalk) maintain private archives. If the official site is down, ask for a verified hash there. Never trust a link without a published MD5.
To download the Mimosa C5c firmware version 2.8.1, follow these steps: mimosa c5c firmware 281 download work
For newer setups, version is available to make C5c radios compatible with A6 radios in Point-to-Multipoint (PTMP) configurations. WISP communities (e