Harnischfechten is an important, yet under-represented part of HEMA. I suspect this is due to the high startup costs associated with getting a full set of functional armor. Still, many of the primary HEMA sources spend a fair amount of time discussing armored combat. If we want to study the breadth of HEMA, then we need to look at armored fighting.
When I started researching the subject, I was surprised by the lack of content for people starting out online. Simple google searches turn up mostly costume or decretive armor. I posted a question to reddit and got some interesting info: Click here for reddit thread.
There is some really good information posted in this thread. First, the suggestion to google SCA and ACL armor turns up a large number of functional (able to take a hit) armor. This is a good place to start and filters out the junk armor in standard searches. But, as the person who suggested it pointed out, these armors are designed for their sports, not necessarily historical accuracy. My favorite link to come out of the thread was a blog put together by a person that has taken the time to assemble a full set of armor from multiple sources (allows for spreading out the cost). My favorite thing about this blog is that although the armor is pulled together from different places, he sticks with a historical time period and theme for the set. The link is here: HTTPs. Finally, someone also suggested http://armourarchive.org/ as a resource. There is some useful information there, but the site has not been updated in some time and many of the links are dead.
There are some good resources out there for learning how to fight in armor and what it is all about. Pursuing the Knightly Arts puts out some nice YouTube content Knightly Arts YouTube Link and maintains a Discord channel on the subject Knightly Arts Discord Link.
I am still not sure I want to make the investment to get into Harnischfechten, but I do find the subject very interesting. I suspect that when/if I am ready, I will follow the lead of the adventuresinswordfighting blogger and collect pieces over time. I did order a pole hammer from Purpleheart to play with some of the forms Purpleheart Pole hammer Link, I will see where that goes as a start.