In general, good items for sweepers are Choice Band (for attack), Choice Specs (for special attack) or Choice Scarf (for speed). It means you can only use one move until you switch out, but they boost the stat 50% which is a lot!
They wouldn't work for your Charizard for example because your moveset relies on using both Sunny Day plus another attack. Same with Garchomp.
There is also Life Orb that boosts attacks 30% but you lose 10% HP each time you attack. Would be good for Charizard.
Leftovers is great for all Pokemon but especially walls with high defense. Combined with something like Leech Seed you can stick around in battle for much longer.
You could also try the berries that reduce super-effective attacks, e.g. a Rindo Berry for Swampert, Charti Berry for Charizard, Yache Berry for Garchomp. They only work one time but it will almost certainly allow you to get a hit or two on the opponent.