The fastest way to get this item on Battle.net is via trading, you can use the Item Price Checker to see how much Isenhart's Parry is worth.
Farming it by killing monsters is possible, check the drop rate table below for more information.
It can also be gambled but the chances are very slim.
Isenhart's Parry drop rate table
The monster with the highest drop chance of dropping Isenhart's Parry is The Cow King on Nightmare difficulty in The Secret Cow Level of Act 1. Farming the monsters listed below gives you the highest chance of finding that item.
Isenhart's Parry variants
Below is a list with different variants of Isenhart's Parry. See the stats of its normal or upgraded variant.