Need a voltage follower buffer for a tube, how do I calculate and select a mosfet?
The biasing of the vacuum tubes looks pretty suspect- I'd expect the cathodes to be isolated from the heaters and for there to be some cathode degeneration at DC (parallel RC). Otherwise you're very much depending on the characteristics of the particular tubes at the particular moment to set the bias point. 

Anyway, a suitable MOSFET for the source follower assuming your relatively low B+ would be the Toshiba SSM3K15FS. It's got similarly low input capacitance, low Vgs(th) and similar other characteristics (except it's in an SMT package and is only rated for 30V).