mavic freehub spacer

I just got a second hand rear mavic aksium wheel from ebay, have just gone to fit a sram 10sp cassette and realized that I am short of 2 or 3 mm spacer that needs to go on the freehub first as the cassette will not fit properly. My Mavic krysium rear wheel has such a spacer already thats how I realized what was missing. Question is does this spacer normally come with the cassette or with the wheel? It wasn't included with either, but the cassette is brand new. Should I contact the seller of the wheel and ask for the spacer, or would a bike store have one lying around that I could have? or should it have come with the cassette?

thanks.