I haven't checked their prices lately, but what about USAKilts and the Scottish Tartan Museum? You know that you'll get quality product from either one.
There's also the British Shop (relatively new sponsor) - less selection than Rocky or Matt, but the last time I looked Ian's prices were fairly close.
I have a kilt suit from the Museum and an Argyll w/waistcoat from the British Shop, so I can personally recommend either one.