You won't find it, it is oral language, often used by TV channels. See this explanation: Letteraly: TELA = SCREEN TELINHA = Small screen TV channels have been using this expression in a informal try to expose their schedule programs. Nowadays it is definitely well known all over the country as a synonym of TV set. 'Telinha' means TV in opposition a 'TELONA', the big screen (cinema screen), but TELONA is not often used.

''-inha'' indicates a diminutive. "na telinha" can signify "on the small screen".. or "on the TV" ..a popular language.