Not a real great deal since the back wheel is 18 inch and almost all motocross rears are 19 inches Google or eBay would be your best bet mate, is it for road legal or purely off road?
Erm I have a tyre in my shed that I was going to put on my bike.. Never got around to it but was really good off road as I had the same tyre on previously..
And no a 19 inch wheel most probably won't fit on as standard.. Maybe if you fitted another swinging arm.. But tbh that would be a real pain in the ass to do so I'd personally just stick with the 18 inch
I'll try and find out the brand of tyre what it was tomorrow and let you know