Try popular online auction sites or specelist Flute stores they may be able to help you.
Where can I find the red flute
It is upstairs in any Pokemon center.
go to a place
The flute doesn't exist in the Crystal version. However, you can find a program for your radio when you update it when the generator is fixed. After it is fixed, go to the radio tower located in Lavender (Not the Goldenrod one) and speak to a man inside. He will upgrade your Pokégear to let you listen to the programs in Kanto. One of them is the Pokéflute program.
Concert Flute is in the C Key
here: http://www.legendarypokemon.net/pass The question was for the crystal version not the gold/silver ones. this code only works for gold and silver.
If you can trade Pokemon from crystal version to gold version my guess is yes
Registeel doesn't exist in crystal version.
you can't unless you have the Japanese version or you have a cheat to get the flute
Gemshorn
hollow cave
There is a version of Prokofiev's Five Melodies, Op. 35, for flute and piano, arranged by Laura Gilbert.