Lot of modules and devices today run on 3.3V and they do not have 5V peripheral at all!
How to drive a 5V generic relay with these?
(are level shifters, boost converters an option?)
Every spare USB battery has a circuit in it that converts the 3.7 volt battery voltage to 5V for USB power. These circuits are called DC to DC boost converters. You should be able to find a variety of them available on ebay. You will not need a very powerful one if all you want to energize is a relay. It will be a good idea to put a diode across the output so the spike from the relay coil doesn't damage the converter.