Fawkes and his feathers

[Eustace_Scrubb]

No: HPFGUIDX 93421

According to Ollivander in SS/PS, the phoenix whose feather forms
Harry's wand core gave just one other feather...which resides in
Voldemort's wand.

Later, Dumbledore tells Harry (is this at the end of COS? I don't have
the books with me...I just hope it's not the dreaded scourge of film
contamination!) that Fawkes is the phoenix in question.  And it's in
COS that we meet Fawkes.

Is there any canon or speculation on how/why a phoenix "gives"
feathers for use in wands?  Whether the fact that only two wands have
Fawkes' feathers is unusual? 

[For that matter, how many phoenixes exist in the WW...according to
Encyclopedia Mythica, the ancient Greek and Egyptian myths tell that
only one phoenix exists at a time.  Is that true in the WW as well?]

Also, how did Tom Riddle come to be chosen by a wand containing one of
those feathers?  (I'm assuming he bought it when he was still Tom, as
I can't quite imagine Voldemort strolling into Ollivanders's and
purchasing a wand in the usual way.  "Yew wood, phoenix feather core,
12 inches...excellent for evil overlord work!")

Is Fawkes small enough to be the "something small" of great importance
in the last two books?

Just some Friday afternoon wondering...

Eustace_Scrubb