Googling squeaky stair repair, I see 2 different recommendations: wedges from
below, and nails from above.

Where are hardwood wedges available? I see the soft ones, about 8 inches
long, but they are soft wood, not something I want to trust to keep the stair
treads in place. Where can I find hardwood ones?

If I go the nail route from the top of the stairs, what's the best nail? 8d
is recommended, but should ring-shank, or resin-coated, or...? It's been
awhile since I looked at nail technology, so recommendations are appreciated.

Thanks,
--
John English