Go into the tree building in Dolanaar and on the second floor you will find a merchant that sells "General Supplies", including 6 slot bags for 5s and 8 slot bags for 25s. There is a vendor that sells larger bags on the south side of Darnassus.
You might also try making a call-out in general to see if any tailors have made extra. Don't pay more than 5s for a 6-slot though, since that is the vendor price.
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Only the exceptions can be exceptional.