where do i get metric nuts for a petrol chain saw?

i brought a chain saw two years again from B&Q and for years i've sent it two a shop to get the blades changed. but out of the blue i saw cheap blades at B&q and thought i'l;l do it my self. Unfortunatly i rounded off the bl**dy nuts that tightened the blade and thought nothing of it just take it two the local handymans and buy two new nuts. Thats for plain thing instead they told me that the bolts were croos threading there 8mm nuts and said you need a metric nut the very old type nut. I must have been to every handman store in the country but still can't find those nuts.where do i get them from