Choose other Pokémon
Perhaps the best part of the Buddy Pokémon system is that you're not stuck with the first Pokémon you choose forever. In fact, you can choose a different Pokémon from your list whenever you want, but doing so will erase any walking progress you've made towards your current Buddy Pokémon's candy. For example, if you're 1.8km towards the 3km needed to get a Candy, choosing another Buddy Pokémon will set you back to zero. Luckily, the app will display a warning if you decide to do this.
The point here, though, is to earn the Candy you need to evolve your Buddy Pokémon to its advanced form and choose another Pokémon you'd like to evolve and do the same. Not only will you finally get the Charizard or Raichu you've been dying to have for months, but you'll also start filling up your Pokédex in the process. The whole point of the game is to catch 'em all, right? And don't forget the Candy can also be used to power up your Pokémon as well (you'll have to spend Stardust, too, though).