You can buy a 'CAM module', which effectively adds a slot to TV sets with integrated Freeview. They should also work with Freeview boxes but it's pointless buying one because they cost just as much as a new Freeview box.
Now that TopUp TV has abandoned its 'card in the slot' service for new customers (in favour of a 'TV on demand' service which requires a special box obtainable only from TopUp TV), the only major terrestrial broadcaster using the technology is Setanta Sports. You can buy a suitable box directly from Setanta for �39.99 but it's probably simpler to buy the whole package (i.e. the box + card + 1 month's subscription) from your nearest branch of Argos, for �49.99:
http://www.argos.co.uk/webapp/wcs/stores/servl et/Search?storeId=10001&catalogId=1500000701&l angId=-1&searchTerms=5321083&go.x=15&go.y=17
Chris