How can I make/find very small on/off 3x3x3 mm switches?
Probably the easiest thing for you to do is to consider using a simple momentary switch that meets your small size requirement and then utilize a separate relay at a location where you have more room to work with. There are numerous examples to search out how you can utilize a single pole momentary switch to turn a relay into an on/off alternate action. Some of these use extra contacts on the relay to keep the relay coil energized when the circuit is in the "latched on" condition. Others add some small signal components (resistors, capacitors, diodes and/or transistors) to add the alternate action to a momentary button and then drive the relay coil accordingly. 

I'll have to leave it to you to figure out just which approach will be best for your application because you left no details about the rest of your design and how the switch would be used.