For the largest selection of general house hold items such as ladders, pumps, paint nad tools in Chiang Mai is defiantly at  Global House with the store covering an area of over 13,000 square meters. It is located on the "second Ring road" about a kilometre from Wiang Kum Kam (the ancient city). 

A Thai owned chain "Siam Global House Co.,Ltd" that is based on the concept of a DIY and contractor supply house like Home Depot in the US.  It is a large warehouse type "cash and carry" store where you can find just about anything you would need to do a build or remodel job.  You see a few construction contractors getting their stuff from them.  Their prices for the home owner are as good or better than you can get anywhere else but for large developers and contractors they often get better prices from other smaller speciality shops that home owners are not able to get.  They deal with lots of suppliers so sometimes, if they do not have what you desire, but if it is available in Thailand they will custom order it with a 50 % deposit.

With each store covering an area of over 13,000 square meters, offers both retail and wholesale customers a wide selection of 70,000 product items.