Just measure your old one , and get a new one that size . Its not hard at all . Call a speed shop , or engine builder , or look online .
Most parts stores are good for the stuff we need a lot , like spark plugs , bearings ,brakes etc . But I would not buy engine internals , carbs , balancers , or any stuff like that there .
BTW , there are several sizes available I can rattle off , 6.25 , 6.75 , 7 ( not common ) , and 8 inch .
And there is internal , and external balanced . Yours is the first type if its stock .