But there are lots. And availability can vary with region. WinTerms were popular in America, but other things were more popular in Europe. A good site to start with is here. parkytowers.me.uk/thin/hware/hardware.shtml The hard part is finding thin clients with card slots. Like these;
I have seen others in other places. Some people have even used "embedded" systems like Lacie storages boxes, or watchguard firewalls. If you find something that works, take a pic and post it to the forums!