While it's technically possible to use a transformer to step 120v up to 240v, this is not likely going to be feasible in your application due to the current draw (likely higher than what a standard 15 or 20A 120v circuit can provide) that would be required to service an appliance made to run on 240v. Can you post a picture of the label from the heater that shows amps or wattage?

Better solution is most likely going to be to run a new 240v circuit from your breaker panel with correctly sized wire and proper receptacle.

Edit (more direct explanation):
Voltage and amperage are inversely proportional when calculating total power delivery. If the heater (for example) needs 15 amps at 240 volts, it would need 30 amps at 120 volts (which would be too much current by upwards of 2x what a standard 120v circuit in your house is set up to provide). You would need to upsize the wire gauge and install a higher capacity 120v breaker, and by that point you may as well just run a new 240v circuit with thinner wire for the lower current.