"I would be happy with either one at this point."
You might not be happy with (20)! As Cisco documentation contradicts itself, either you need further verification that (20) would support the HWIC or you'll want no less that (24). (Ideally, Cisco would be the best source to verify what's the minimum required release, but as router is EoL, they might not provide such verification. Or, also great would be someone who has actually used that HWIC while using [20]. BTW, I recall [?] I've used that HWIC in 2811s, with 12.xT, but it's been so long ago, I cannot remember what release was used.)
Also, BTW, a key factor in finding an IOS version in supporting those HWICs is the "or later" release and/or IOS version. Also, as you might be aware, don't believe you need an IOS specific to the 2821, I recall same IOS images support the 2811 and 2851 too, unsure about the 2801. Basically, look at your running IOS filename, if starts with something like 2800 (near the beginning), that other IOS images with similar names might do. (I mention this, to hopefully, making finding a newer IOS file, a tiny bit easier.)
BTW, when seeking your IOS, you might also try contacting some of the, on-line, software and/or network equipment sellers. Those that sell Cisco routers and/or Cisco IOSs, might have an old 28xx IOS sitting on their shelf.
From your other posting, I understand you do have some IOS version running on this 2821. If true, for teaching purposes, if you cannot find an IOS image to support your HWICs, some alternates you might consider . . .
If you have a VLAN capable switch, you can create multiple routable subinterfaces on your built-in Ethernet interfaces.
Or, if you can find any of the L2 Ethernet cards/modules, that your current IOS supports, you can create one VLAN per L2 card port and have routable SVIs. I also recall (?) there was also a L3 Ethernet module that could be added to these routers (basically a 3560 on the module card).
From the prior paragraph, if you're wondering why did Cisco even provide the FE port cards, those cards supported the same software features as the built-in Ethernet ports, that the L2/L3 modules cards did not. (Conversely, though, in real-world usage, the 2821 didn't have the forwarding capacity to support wire-rate on its built-in Ethernet ports, let alone adding more such Ethernet ports. The L2/L3 modules could do wire-rate between those ports, but also would have performance issues to/from the rest of the router. In a teaching environment, I assume the router's performance is a non-issue.)
Lastly, although I very much believe doing some hands-on training on actual equipment is great, you might also consider using something like PT for teaching many of the basics, and then using actual hardware to reinforce some lessons on "real" hardware.
