Have a look at somewhere like Halfords there maybe a inbetween size .
to get it out without you may be able to use a small pair of stillsons or try knocking it round with a chisel as a last resort knock a bolt in to it and weld it to the plug the heat of welding helps as well .
if you damaged it getting it you can get a new one from grahams plumbers merchants or similarities the past I’ve used one with a external square.