In most cases, it's going to be the barrel. This may not always be the case - it all depends on WHICH .22 rifle you're referring to. Some models may have stronger receivers than barrels - others will...
Try MidWay USA, E-gunparts.com, Brownells, gun shops, gun shows, pawn shops, want ads, for sale ads, estate satles, garage sales, on line auctions, local auctions.