Bought a 230V air conditioner, but my apartment is 110V. What to do?
No, there is no magic doohickey to solve this problem.  Not least because it introduces a bunch of other satellite problems. 

These aren't like "Windows vs Mac" 2 incompatible flavors of the same thing  240V is actually more powerful, so an A/C unit would not be made 240V unless that was necessary to power its functionality.    So a 240V A/C will probably overdraw a 15A circuit even if you used the (non-cheap) tech to convert.  

The right thing is to take it back and get an appropriate A/C.   How did the salesman not have this conversation with you and make sure you had 240V power available?  Don't do business with them again, they stink at their job!