Mimiron's Head is a very rare drop from Yogg-Saron in Ulduar. It will only drop if none of the Keepers is assisting during the encounter, giving it the same prerequisites as Alone in the Darkness. It can also be listed on the Black Market Auction House.
Here you can buy Mimirons Head for EU or US Servers.
You'll get guide for getting Mimiron's Head
You'll be able to get Mimiron's Head mount
Additional budget for Character transfer or Character boost to max level. BMAH Services in Shadowlands now requirement to have a max level character on server, so Trial character will not work.
Free time on evening, if you choose selfplay option