It was advertised in the for sale section on here - took a bit of faffing about to get it working as I had to change ATV versions from 6 to 5.3 but works spot on now.
Mine is a HMP dvb-c tuner and you can buy them on eBay for around £38+ P&P but the only issue is the driver not working on ATV 6 so you'd need to have ATV 5.3 installed to use one but there's other dvb-c tuners out there too like the Sundtek ones which I have heard work fine too but these are around £60 I think and I have no idea of their compatibility.