I called Springfield Armory a couple of weeks ago. They said that extra barrels, slides, etc. may be available in a year or so; all of the ones that they are currently manufacturing are committed to producing new, completed pistols.
After learning this, I called Irv Stone at Bar-Sto and ordered a .357 SIG barrel for my SA XD40. The cost is slightly less than $200 and will be shipped to me in 10 days - 2 weeks.
Just as an aside, I also have a .357 SIG barrel on order from Bar-Sto for my .40 caliber Walther P99; I should have this one in about 3 weeks.