Around here pretty much any auto parts store will have one or something that will work or if you insist they will order one.
Ask for a BR6S by NGK.
If they don't have that ask for a BR5S, BR7S, BRP6S, BRP5S, BRP6HS.
If they don't have any of those then ask them to order the BR6S.
If you get a new plug and that helps but the problem comes back then I can guaranty it is the ignition coil.