I'm posting this because I actually spent hours looking for a bag seller that sold larger than 4 slot bags!
A 6-slot bag called a Tanned Leather Rucksack can be found in Nettleville Hovel at the bank on merchant Baily Dowden Loc. pos654, neg24, pos342. Price 6sp, 40cp.
Also Merchant Caryn Swell under a tent Loc. pos611, neg26, pos383. same price