Hi I`m Jack ,different tech. here
I believe you got your ohms law backwards here .
Your source is 120 volts and that's not variable so while it is correct that a 250 ohms load under 200 volts will give you approx. 800 milliamp (.8) , your source is not variable unless you where to use step up transformers.
Now if you use a constant voltage source like 120 volts and then a step down transfo 240/120 when oversea then you have a constant voltage source .
Now usually device will draw a specific amount of currents depending on the devices and the object of a power source is to be able to supply (in voltage and current) the device that loads that power supply .
For example if your device draws .8 amp then you want a power supply capable of delivering above that under a constant voltage not a constant current , that does not exist , the load determines the current draw not the power supply .
For example what you would need is a power supply that can deliver 1 amp and above if your device needs .8 amp
May be easier if you just said what it is that you want to power up and what voltage it works under , is it AC voltage or DC that you need ?