I have an oil electric lervia heater?

I doubt you will find an exact match... or anybody prepared to sell just one wheel...

Us google
select Images
Enter Castor Wheel and look at the pictures.

Buy a set of four - knock the three you still have and fit the new ones - best if the new ones have a threaded stud for easier fitting.

Amazon sell them too - a pack of four for £4:17 carriage free.
Fixman Twin Wheel Castor (threaded 40 mm long 8 mm thread)

Stay Warm...

Good luck...
January 2015