No offense but you need to look at who can use a Brais...MNK, RDM, WHM, THF, PLD, BST, BRD, DRG, and SMN while Slacks are MNK, WHM, BLM, RDM, BRD, RNG, SMN.
Yes for some it would be better to wait but not everyone can use both.
Hmmm...Edit: These are dropped of Yagudo Mendicants as well as crafted.
You can buy them from the NPC at the end of the dock in Windurst Port. If I remember correctly, they were approximately the same price as in the AH at the time I checked.