On www.yallo.ch (does cheap sim cards – text to anywhere in Europe for 15 Cents, a call to any other phone for 22-25 Cents / minute in most countries – see list of countries here: https://www.yallo.ch/kp/ext/pdf/yallo_tarif_overview_e.pdf
Any electrical shop such as Fnac, Fust, both of which are in Balexert and in the city centre, can advise you on options.