I've moved recently to a house with quite a few Pahutakawa, did a cut back of one in ground around December.
Back budded well
One in flower
I think one of my cuttings has rooted but will leave to Spring as heading into winter here in NZ.
Sorry no thought on where to buy overseas but happy to hear people keen on New Zealand natives.
Metrosideros can make great bonsai. They grow back after pruning very well and tolerate root pruning very well. Can even develop aerial roots if conditions are right.
Can't help with sources for California.