Kung wala kang makuhang bolts (or info about the size) sa service center sukatan mo na lang.
8mm diameter bolts (M8) are used for TVs 47" and larger. Smaller TVs use M4, M5 or M6. Yung lalim (measured flush with the back of tv) pwede mo sukatan using palito or bbq stick. Add the thickness of the mounting plate and any other spacer to get max bolt length.
So kung 31mm ang calculated max bolt length, you have to buy 4 pcs M8x25mm and 4 pcs M8x30mm bolts, plus 4 lock washers. Try the short bolts first if it threads nicely. Use the longer ones kung parang sobrang kaunti yung naka-thread sa loob.
PS wag ka bibili ng sobrang habang bolts baka lumusot sa screen, hehe!