In truth, it is almost impossible to buy anything 'truly' anonymously, especially on the internet.
Probably, the most anonymous way to acquire Zcash would be to use a mining pool that only runs over Tor, as a hidden_service .onion - always connecting your mining software to the pool though Tor (of course) and only ever connecting your Zcash wallet via the Tor network.
Encryption, arguably, can only ever buy you more time! Consider that to be truly anonymous you must hide all of the following knowledge forever : Who? What? When? Where? Which? Why? and How!