Hi Bill,
welcome to the NeuroTalk Support Groups.

I've no idea where you could get one really, but you could perhaps grow your own.

What exactly did it taste like, do you remember?

Some bakers make their own yeast 'plants'. I came across one who used potatoes, raisins, salt and sugar.

There's also kombucha, but you didn't mention tea as an ingredient.

Only thing I'd be worried about these days is consuming some sort of icky contamination. I didn't think about that as much when I was a child.