Does anyone know a reliable source from which I can purchase

>>273650
There's lots of options for mATX if you search for htpc or slim cases. If you really need to accommodate a full size ATX motherboard, the only readily available cases I could find are the Silverstone Grandia line. But those are essentially mid-tower cases turned on their side, not a low profile / half-height you probably want.

>>273718
OP clearly wants a full-height desktop case, as shown in OP.

You might not remember them, but they absolutely were a thing. TFT probably killed them, because without a big fat CRT, there's no real reason to shove the PC under the monitor.

>>273781
Aside from the fact that it saves space and can elevate a monitor into an ergonomically favorable position (lots of computer users suffer low-grade neck-strain from craning their head slightly).

>>273718
No, I'm trying to build a gaming rig. I have unlimited vertical space, but because I'm 6'8" and live in a small apartment, my horizontal space is limited. I need something I can set on TOP of the desk, or on a shelf above the desk, and not have to constantly knock my knees against.

>>273863
It doesn't save space when compared to an all-in-one or a slimline parked behind the monitor or a mini-itx bolted to the monitor's VESA mount.

Which is why no-one buys them anymore.