Where can I find weak Pixelmon?
You cant can't necessarily find weak pixelmon in a certain area, because the pixelmon that live and spawn in said area have randomly generated levels according to their current evolution stage and which pixelmon they are themselves. (Example: If a Weedle spawns In a plains biome, its level will be randomly picked from in-between 2-12)
My advice would be to do a lot of searching around for weak pixelmon to battle. If you still cant find any weak pixelmon, your you're better off creating a new world to play pixelmon in.