Here you can download some sprite packages for both frames of all 493 Pokémon from Pokémon Diamond, Pearl and Platinum, complete with transparent backgrounds!

 

Pokémon Diamond/ Pearl
Pokémon Platinum
Frame 1
DP1
PL1
Frame 2
DP2
PL2

 

You could use these for making sprites or anything else you want to use them for: no permission necessary to host and re-distribute them. Also I have some Heart Gold/ Soul Silver spritesheets for your convenience:

Pokémon Heart Gold/ Soul Silver
Male
Female
Other

Finally, you can download some spritesheets/ zipped packages for older games:

Pokémon Red/ Blue
Pokémon Yellow
Pokémon Gold
Pokémon Silver
Pokémon Crystal (5 sheets as there is a sprite for each frame of animation)
Pokémon Ruby/ Sapphire
Pokémon FireRed/ LeafGreen
Pokémon Emerald