How do I get the other two starter pokemon?

if you want everything legitimately, you'll need to trade with someone else, it's plain and simple.

It's the same for several other pokemon, which aren't available in Red but are in Blue (or yellow).

Some pokemon also evolve only during trading.

This is to encourage people buying the other game as well (and another gameboy) this is called 1 game for the price of 2.

I believe the yellow version, you can get them all. You start with a Pikachu, then through the game get given, win or find the other primary Pokemon: Charmander, Squirtle and Bulbasaur. Like others have said if you didn't want to buy a new game, you'll have to trade with other people, or between your devices with your other games.

Unfortunately in red and blue you only get that one you choose.