Where do you purchase your classroom IT equipment?

I've done this in the past - you don't have to use updated machines (or even functional ones), to go through things like laptop, printer, or PC tear down activities. For some students, this may even be a bonus because of the fear of "messing something up" as a result of learning how to do proper system tear downs. Or even if the machine is working, if it's a $50 machine that gets popped in the lab, well, no harm done.

It's also amazing how many people want to give you old hardware that they no longer want. I've had to turn people away over the years, lest I become a dumping ground for old tech. Luckily, we also have a good recycle spot that does tech buyback.
 
I've done this in the past - you don't have to use updated machines (or even functional ones), to go through things like laptop, printer, or PC tear down activities. For some students, this may even be a bonus because of the fear of "messing something up" as a result of learning how to do proper system tear downs. Or even if the machine is working, if it's a $50 machine that gets popped in the lab, well, no harm done.

You got it! And students can take parts from a couple of dead boxes and make a running box. When I was in the classroom, we would have "Game Day Friday" but in order for them to play they had to first make the computers run and then network them together. Amazing how fast they learn in that situation.
 
Here's another suggestion: your local IT enthusiast forums often have want / classified ads.

In the Netherlands, https://www.tweakers.net is a large enthusiast group for professionals and hobbyists. Their "V&A" ("vraag & aanbod" : requests and offers) often has old server hardware, which was previously used in a homelab or to run gaming servers. That's where I got a decent Dell R710 last week.