I use this website for sprites:

http://pokemondb.net/pokedex/national

Just go to the Pokemon you want, click on it to go to its info page; there's a link near the bottom that leads to its sprites, so click on that and on the sprite you want. From there just copy it and paste it into the post. You might have to change the "mode" that your post is in to get the sprite to show up. Good luck!