Pokémon HeartGold and SoulSilver/Special Pokémon

Swarms
Various Pokémon can be caught by listening to Oak's Pokémon Talk on the Pokégear, and then going to the route indicated by Mary.


 * - Route 13
 * - Route 47
 * - Mt. Mortar
 * - Route 35
 * - Dark Cave
 * - Route 38
 * - Route 32
 * - Route 44
 * - Route 1
 * - Vermilion City
 * - Route 34
 * - Route 9 (HG only)
 * - Route 9 (SS only)
 * - Route 3 (SS only)
 * - Route 45
 * - Violet City
 * - Route 3 (HG only)
 * - Route 19
 * - Route 12
 * - Route 27
 * - Viridian Forest
 * - Route 25

Gift
I actually have no idea apart from that you can receive pokemon, from people if you talk to them such as Steven is Silph Co.

Trades
Vermilion Gym Leader Lt. Surge will trade with you, but only if you meet the requirements for the Gym Leader Rematch. As his favorite Pokemon is Pikachu, the most likely Pokemon he wants will be Pikachu. In return, he will trade you his French Pikachu (ie. your Pokédex will contain French-language data for Pikachu).