User blog comment:Wardie1993/Ideas for where I can get an armour piece enchanted with "Fortify Heavy Armour"?/@comment-6881629-20150725183709

It's random keep an eye out at merchants and in loot for items with the word knight at the end, like "Item of the Minor Knight" is a weak version or "Item of the Peerless Knight" for the strongest. I don't think there are any in set locations, if you desperately want the enchant just keep going round all the merchants by the time you get to the last town the first one you visited should have new stock.