There are 3 ways to get one.
1.) After getting all 16 badges, Professor Oak will give you a pass to Mt. Silver where you can fight Red, and if you win, you have an opprotunity to get starters from Kanto/Hoenn.
2.) You can use Pal Park to transfer a Hoenn starter from Ruby/Sapphire/Emerald.
3.) Trade w/ someone who has already defeated Red and received a Hoenn starter.