The ones I got have their own power supplies, I don't have rails, so I was pretty careful about this.

I'm not totally sure about this, but I think even ones with rails, you can often take the adapter card off the motherboard that connects them to the rails and put in your own power supply. In practice, it would probably be way easier to buy something thats already configured to have a power supply instead of use rails for power, there are plenty out there.