The USB ones are expensive, if you are certain there is no jack sockets and its an old PC then just buy a cheap PCI card such as click here you need Win 95 or above. Or pop into your local computer shop they probably have a cheap one for about a tenner.
The one I gave a link to installs internally in a PCI slot, they are the white coloured slots you will have 2, 3 or 4 of them. They are not differcult to fit. If you really want an external USB one then perhaps this click here but as you can see its more expensive and it only supports win 2000 or XP, if you don't have either of them then you might as well forget it as I'm not 100% sure but I think any USB one needs at least windows XP.