Dc isolation (transformer?)
First, there are no such things as DC transformers.  Transformers inherently work on changing magnetic field, which doesn't change if you put DC thru it.

What you are asking for is called a "DC to DC converter".  There may be modules you can get that do 1:1 with isolation.  You'd have to look around.

However, this smells like the wrong question.  It's a bad idea to ask about a particular solution you've decided on for whatever reason.  If you're asking, you probably aren't aware of the various possible solutions.  Explain what you are really trying to solve.

It sounds like you have a ground loop problem.  There are various ways of attacking them, with a DC to DC converter not usually the best answer.  If you've got two separate things to power, why not just use two power supplies to begin with?  What connections go between the two units?  How far are they?  What are the nature of the signals?  Can these signals possibly be transformer coupled, or opto-isolated?  There may be other ways too.

Step back and explain the real problem.