There are three regional merchants for each region, with one merchant located in each of the competing nations: Bastok, San d'Oria, and Windurst. These merchants only have goods to sell when their nation controls the importing region, as shown by the Region Map. For example, if San d'Oria controls the region of Aragoneu, Antonian in San d'Oria will be the only regional merchant for Aragoneu to have goods for sale that week. If the Beastmen control a region, that region's goods will not be available at any of the three merchants until a city gains control.
Your reputation will affect the pricing of these vendors - the higher your reputation, the lower the price.
To check which regions a city controls, check the Region Info (by going to the menu and selecting Region Info or typing /rmap ). The regions are represented by the icon of the city that owns it. If it is controlled by Beastmen then it is represented by a gray dot.
There is an exception to this rule: if the Super Kupower Bountiful Bazaar is currently active due to the acquisition of all the Mog Tablets, all three areas will have ALL of their regional merchant NPCs open for business, so you only need to stop at one town, should you need to grab ingredients from more than one region!
Many of the goods sold by regional merchants are also sold elsewhere. As a general rule, the lowest price for an item sold by an NPC is from a regional merchant if the character purchasing the item has high reputation in the city where the regional merchant is located.
The regional merchants added with the Chains of Promathia expansion are a bit different from the rest. The merchants representing the region of Tavnazian Archipelago only sell items to characters who have advanced through the missions of that expansion; a character must be at chapter 4 or above in the Promathia missions to buy goods from them. The regional merchants for Movalpolos sell to anyone, but also within the dungeon of Oldton Movalpolos is the merchant Bartabaq, who sells some of the same goods as the regional merchants for Movalpolos, but will only sell to characters that are from the nation which controls the Movalpolos region that week.