There are a total of two Ways.
Each requires doing a lot of one mod however.
You can A:
Breed imperial and industrious bees and use iodine capsules (for rain) or dissipation charges (for sun) in a rainmaker.
B:
Do tons of Thaumcraft and Thaumic Tinkerer Research, until you unlock the weather crystals.
Make them.
It uses 1 crystal per change, and to get 3 per Quicksilver+Amber+Fire/Water shard.