You can buy an Oppo A57 on Lazada for 1,497 Bt or an Oppo A83 for 1,589 Bt. You should be able to find similar prices at Tuk Com or at the phone shops in the malls like Bic C Extra, Pattaya Klang and 3rd Rd or on Sukhumvit., or at a Lotus's on Pattaya Nua or on Sukhumvit. Those stores also have their own phone shops so you can compare prices.
If you don't want to buy a new phone I'll second the advice to go to Tuk Com to buy a second hand phone. Be ready to spend about 2000 Bt on one from Oppo, Asus, Xiaomi, Samsung, One Plus, Sony, or Lenovo. Ask for a 1 year replacement guarantee, which you won't get, but if they give you at 6 month guarantee then buy.
Notice these are all Android phones. If you want an iPhone, which are way overpriced in my opinion, you are on your own.