i have 99 cav 2200 3 speed auto where can i get parts to upgrade this trans

For final drives (differentials) best place is a salvage yard.

Shift kits (Superior, TransGo) Locally owned Transmission shops, that carry TranStar Industries parts, or any racing trans shop.

Raceing quality rebuild kits, any trans shop that carries Alto Red Eagle/kolene friction kits, i.e. race trans shops.

Torque converters, any race TC shops.

Any of these shops can be found locally or through any internet search engine.