You can simply apply a trial Enterprise Plus license (I don't believe Enterprise is offered any more) to your hosts and turn on DRS. You will probably need to speak to your VMware account rep for a trial license. There is no Enterprise Plus license for vCenter, only vCenter Standard (which you probably already have).
Everything is licensed at the moment with standard licenses. I did attempt to roll back to the existing trial but the period is already up.
I have been calling VMware sales for the last 3 days now, never get through and they ignore the messages that you can leave for a call back. I will have to try via a partner I suppose..
If you're a small customer, a partner is probably going to be your best bet.