There are 2 electronics shops on Theprasit road that may have a transformer.
this one the most likely
https://www.google.com/maps/place/ส.+อิเลคทรอนิคส์+อะไหล่+บ้านหม้อ/@12.9087091,100.8877886,3a,77.4y,-2.5h,100.27t/data=!3m10!1e1!3m8!1sWQ3ox-uBmW644juczz1JGQ!2e0!6shttps:%2F%2Fstreetviewpixels-pa.googleapis.com%2Fv1%2Fthumbnail%3Fcb_client%3Dmaps_sv.tactile%26w%3D900%26h%3D600%26pitch%3D-10.266866518036409%26panoid%3DWQ3ox-uBmW644juczz1JGQ%26yaw%3D-2.50454992849396!7i16384!8i8192!9m2!1b1!2i48!4m6!3m5!1s0x3102973154de224f:0x1e3b1145e74a8858!8m2!3d12.9088155!4d100.8877187!16s%2Fg%2F11h0ykq4tf?entry=ttu&g_ep=EgoyMDI1MDQwMi4xIKXMDSoASAFQAw%3D%3D
and another shop on the same side of the road a bit further down towards the beach normally has reels of speaker /hifi wire out front they may have something too.