Most provinces have access to the major suppliers, listed below in no particular order.
Note: I would not suggest ordering unseen, online, for items you have not physically examined as descriptions/dimensions etc are not always accurate.
HomePro - mainly whitegoods, TV's, lighting, paints, tiles and furniture though a reasonable stock of plumbing and hardware supplies may be found at larger stores. Customer service is normally excellent, delivery, installation and warranty service is efficient. A 1st stop and good yardstick when shopping around for large ticket items. For raw building materials, shop elsewhere.
GlobalHouse - similar to above although a huge range of raw building materials, cements, steel, wood, pipe, etc is available and their range of general goods is generally of more affordable quality. Despite recent store upgrades, aircon and other modernisations, customer service is still below par.
DoHome - Founded in Ubon Ratchathani as Ubon Watsadu this progressive company has expanded nationwide into one of the largest building and home supplies outlets ... if they don't stock it, no-one else does. Furniture tends to be aimed at lower end markets but all other ranges are well covered. Their warehouses dwarf any of the above and like GlobalHouse, there is a separate raw materials warehouse at the rear for public and trades alike.
ThaiWatsadu - Same corporate owner as Baan&Beyond retail, therefore many household and tiling products are found in both stores. TW used to be the big go-to supplier for everything though DoHome and a renovated GlobalHouse are giving them a hard run.
Boonthavorn - Perhaps the best saved for last mention, though a raw materials outlet it is not. Boonthavorn is unique, a privately owned company with magnificent stores and landscaped park-like gardens with onsite accommodation for very loyal staff. The owner is benevolent and revered by all who him. Apart from looking almost resort-like with water features, koi ponds, high ceilings and indoor trees, the staff are exceptionally well trained. That alone is a big plus.
Tiles, predominantly imported or top quality local product is their forte and the displays are stunning. White-goods from the major manufacturers + many modern design kitchens are all displayed. The design/quote service is 2nd to none. We purchased one of our 2 kitchens here and the whole process including installation was the most professional I have experienced EVER.
Furniture and lighting, is of a high quality, well represented and displayed well. Bathroom equipment incl western shower screens, spa's and the latest imports are all available. Boonthavorn have a general hardware section which is extensive although not always the cheapest for common items.
The Chiang Mai store is worth a visit to simply view the overall design and huge manicured park at the rear. Minor details like automatic water spray wc's in the squeaky clean facilities, a food court and Amazon coffee shop all make for an interesting visit.