Optrel and cheap are not words that go together. If you mean the covers they are not expensive as they are standard but if you mean the self darkening LCD unit it may be as cheap to replace the helmet.
You need to look for an Australian supplier but there are American ones selling a 5 pack for $20 and they are available on ebay for similar price over here.
Thanks, here in Australia lenses are quite expensive, this helmet isn't very common over here, I noticed on ebay USA you can pick them up cheaply but shipping may be a bit much.