Just for clarification, what do you mean by mutton? In some parts of the world, it refers to goat and you're not going to have any trouble finding it - as cabrito - in either SA or Aus.
Mostly tho the word refers to older sheep, and that is going to be harder to find.
I'd suggest looking at Halal meat markets, both goat and lamb are very popular meats in the Middle East. I can't recall ever seeing anything labeled mutton in Halal meat markets in Houston but it would be worth looking.
I'd also suggest you post on the Austin board.
I can't recall ever seeing meat labeled mutton in Houston except on some restaurant menus, where it referred to goat.
Thanks for your reply. You are correct about mutton being an older sheep. Southside Market in Elgin advertises mutton, but I'm not sure if that is not lamb. I will try the Halal markets. There used to be a lot of mutton raised in the Texas Hill Country, but no more. An older male goat is also referred to as mutton. The problem is, that ranchers used to castrate male sheep and goats and raise them for the wool and mohair. Very few of those guys left. Now, sheep and goats are just raised for the meat so the young males are sold young for meat.