44 Mag - always available.
40 S&W most available auto round.
45 ACP second most available.
9mm still not regularly available, but at least starting to show up more than once every other month.
38 special pops up at Wal-Mart every few months.
357 Mag - haven't seen it on the shelf at Wal-Mart in a year at least. See some premium stuff at other sporting goods stores.