I want to fill up my pokedex but I don't know how to get the different forms of Rotom. Is it even possible to do on Black/White?
There isn't a pokedex entry for the different rotom forms.
But you can change the forms.
Take rotom with you to the Department Store in Route9. In this store, there is a room with multiple storage boxes. Interact with this box and you'll be able to change your Rotom's form.