HamSamich is mostly right.
There IS a core card, and each site is SUPPOSED to accept it. However, the core card is only HALF of the actual RCT certification. The 2nd half is site specific.
The RCT CORE test is BEST taken where you are going, but there are a few companies that will give it to you. There is no guarantee that the DOE will accept those cards. But they MIGHT.
If you are in a situation where the site requires you to have a core card BEFORE you apply, then you can go that route, just understand they may make you take it AGAIN when you get there.
If you are real desperate, contact me.