From what I gather (from the documentation) there are a couple of cheap options available. First, digium used to make an S100U which was a USB device that offered an FXS port. Despite my best efforts I have been unable to find this for sale anywhere on the web.
Also, I believe that Quicknet Internet Phonejack cards can be used for their FXS capability. Again, I haven’t been able to find any place to buy these online either - not on eBay and Quicknet’s online store seems to be out of action.
Does anybody else know where CHEAP FXS interface cards can be obtained?