How do I select a transformer for superhet?

I've recently been learning about superhet and frequency mixing and wanted to start tinkering. Specifically I'd like to try using two heterodynes in series to first frequency shift then uninvert the original audio, sort of like an analog frequency shifter.

To do this, I'd need a frequency mixer. I've been looking at a ring modulator (like https://upload.wikimedia.org/wikipedia/commons/1/1b/Diode_DBM.png) which should require 4 shottky diodes and 2 center-tapped transformers. I've had difficulty locating an affordable transformer, with good enough fidelity for audio, that also includes a center tap.

I have a few questions:

• Where can I locate affordable, good-enough-for-audio transformers?
• Is the ring mod approach good enough? I see there's also a gilbert cell.
• Any general advice for someone just starting a project like this?

Thanks!