If you are OK to mirror at 10 BT (i.e. 10 Meg) then you could see if any one you know has an old ethernet HUB lying about. Some people use these as a way of monitoring the the ethernet traffic.
For a cheap switch that will port mirror at 100 BT you could try to get a second had CISCO catalyst switch. I got one of these (catalyst 2912) of ebay for £27.65. That should have a command to port mirror.
I have another switch that I bought new and that was the Dell PowerConnect 2716. I bought the 16 port version as it was cheaper than the 8 port (2708). The 2716 does do port mirroring but I'm not sure about the 2708. You need to check the Dell Website for your location for prices of the switch, but at present in the UK its £77.55 inc VAT and delivery.