OP noted SIS shifter so I'd assume indexed. As long as you get a long-cage derailer and it is 2:1 cable ratio (the most common but some SRAM have 1:1 and won't work with Shimano indexing) you are fine. You can pick up a Tourney at Niagara Cycle Works for $10 plus shipping. I'd check eBay first to see what you might find (and older Deore might be reasonably priced and will be a little smoother and more durable). Then I'd see if your LBS has some take-offs that are still in good shape before buying new online as you'll end up paying almost as much for shipping as for the part.