Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Kuching to Barcelona as you normally would.
Kuching (KCH) is the airport you will fly out of if you book a flight from Kuching to Barcelona. You should not have trouble finding flights to Barcelona from Kuching; the airport handles an average of 0 outbound flights to Barcelona per day. You should also have plenty of airline options to choose from since 0 airlines have flights from the Kuching airport to Barcelona. It is only a 7.4 km journey to Kuching (KCH) from Kuching’s city centre.