I called them and gave the part number. They told me that it is on backorder, with no anticipated available date. I felt somewhat hopeful, at least they didn't say it is discontinued completely!
They took my info for an order, but here's the strange part - they didn't ask for a method of payment. I asked if I will get a call when it is available, and she said no - I should call back in three to four weeks to see if they came in. I instantly went from hopeful to skeptical.
In the meantime, Midway replied to my email, offering the Recoil Spring Guide Assembly, S&W part no. 263280000, but the photo on the order page appears to be the rail and plunger only - no spring, no joy found. That part's on backorder with a 4/16 anticipated availability date. I think I'll order that just to have it on hand.