Doesn't the SB Live have digital in (pin 6 on the expansion header?)? SB Lives must be by far the cheapest and most common sound card with digital in.
Judging by Ebay you'll have a better chance at getting a TB cheap than a Terratec. The Montego II Plus or the Montego II Home Studio come with a daughterboard with digital in and out. I have a vague memory that the signal from the daughterboard to the main board input is I2S, not S/PDIF, so it's not trivial to add a digital input to a regular Montego II or Quadzilla.