Where to get heavy duty wheel bearings?

Does anyone know where I can buy heavy duty wheel bearings for my Murray cruiser bike, front and rear? I'd like some that I know will hold up better to the higher speeds and longer driving distances. Perhaps an axle/bearing assembly I can just bolt right on? I haven't had much luck searching Google yet.

Thanks!

There is a heavy duty wheel kit for your back wheel that has sealed bearings and an integrated sprokets. But if you just get the kit, you have to lace the spokes into your rim yourself. Spooky Tooth sells the fully assembled wheel for 130 bucks. I am still waiting on mine, but I am sure it will be ideal for my 26-inch cruiser. P.S. it has a built in friction brake with cable included. Brvnkaerv