Amazon do bike alarms. $16? Kind of intended for motorbikes(?) but when i turn then both on - one wedged under the seat and another less permanently attached - the conversations generated guarantee me onlookers (bar/ cafe) almost look after the bike for me. Decent lock, too. Drop the chain off and whack it out of gear (it hurts if you forget). If im inside and i see someone checking out thd bike i 'test' the alarm.