240v baseboard heater on a 120v circuit using a step up transformer ?
Heaters are cheap. There'd be no reason to do this!

An electric heater is one of the simplest appliances in the world -- so it is inherently very low-cost.  It's not cheaply made, it's just simple enough it doesn't cost very much to make one. 

It will be cheaper to buy the right heater than adapt the wrong heater.

I imagine you got this thing gifted to you, and you have no idea how much heaters actually cost.  Go find out; the typical baseboard heater is a Cadet, and they are around $50 for a big one.  You can't buy a step-up transformer for that, you are in peril of spending $150 to change the voltage on a $50 heater. Don't do that lol. 

Now if you're stuck with the common receptacle circuit in your house, Those circuits are not magic.  They cannot carry unlimited amounts of power, in fact USA circuits are particularly limited to a measly 1440W, or just one of those portable heaters.  So it wouldn't matter if you got a big baseboard heater that is 120V, you couldn't hook it up anyway.  

Contrast with a properly wired 240V circuit, which can carry 3840 watts, or almost 3x as much, that is why baseboard heaters are mostly 240V.