If a circuit only has a current source and no voltage source where does the voltage come from to supply the circuit?

An Ideal current source will produce whatever voltage is necessary to permit it to deliver its specified current.

An ideal voltage source will deliver whatever current the rest of the circuit requires when it is delivering its specified voltage.

Real current and voltage sources will have limits on the voltage (for a current source) or current (for a voltage source) that they can deliver.

These are idealized elements. Neither voltage sources nor current sources exist in reality. Reality has generators and galvanic cells etc.

You can model a galvanic cell as either

  • a voltage source with an internal resistance in series

  • a current source with an internal resistance in parallel

Of course you can always add more things to your model, but these two options are the minimum. If you drop the internal resistance connected the right way, you aren't talking about real world things any more but you are talking about your modeling.

And that's what these circuit lessons and practice is about. Learn to understand the modeling. So you can build and understand models for real world elements.

I think you can now easily understand what to do with R2, and what the current source and R1 can be transformed into afterwards.

A current source does have to have a voltage source. But, because a current source is designed to supply a fixed amount of current, we can ignore its inner workings and just focus on its purpose: to be a current source.

If you think about it, we treat sources as ideal a lot, and it works out just fine. If we couldn't do that, a voltage source in a circuit would have to include all the components that allows it to output that particular voltage. And if it were plugged into the wall, it would have to include a circuit that went all the way back through transformers and many miles of wire to the electricity source...