How to buy 1g of enzyme that is sold in units/mg?
Sigma Aldrich defines their units as:


  One pyrogallol unit will form 1.0 mg purpurogallin from pyrogallol in 20 sec at pH 6.0 at 20 °C.


Assuming you're doing a biological imaging experiment, these activity values tell you much more about what you're getting than mass.

From a casual internet search it appears that HRP is pretty expensive. I would consider whether you actually need so much of it for your research.