That would depend on what you want?
First of all why, dont you bring some of your stuff with you, phone and computer wont have an issue working in Thailand
But I you want to buy, there are a lot of reputable places to buy good gear, Apple has just opened an official store at IconSiam and there is also Studio7, I have had stuff from them before
All the other makes and Brands are in IconSiam and also in Emporium, Siam and other major shopping places.
As for TV's the places above also sell good makes, but you can also get named TV's from places like Big C and Tesco Lotus