Where can I find an unswitched ethernet hub?

If your application for a hub is "inserting a protocol analyzer into a network connection", the modern answer is a switch that features "port monitoring" or "port mirroring", in which one switch port is configured to emit all packets to and from another specified port.

For example, the Netgear Unmanaged Plus switches provide this very function, at 1Gb and 10Gb. The Netgear GS105Ev2 is a 5-port 1Gb switch in this line, available for $40 on Amazon.

Amazon to the rescue.

Unfortunately it's a special product now (people who want them really want them and can't use something else) so the prices may be higher than you expect.

EDIT:

Also you will NOT get a gigabit hub. You won't find PCI-E video cards to use on a 386 motherboard, either...:)

If hubs are hard to come by, another option might be to buy a Wi-Fi router that runs dd-wrt, which it should be possible to configure port mirroring on.

Another answer from Amazon : Dynex DX-EHB4 - Hub - 4 ports - Ethernet - 10Base-T - DC power - external, 1 new from $22.99 and 16 used from $1.84.

Just asking for Ethernet Hub in Electronics will get you 355 products (most of them actually switches).
You may reduce the results further by price etc. to narrow the research.