Can you please help me find a light ball in pokemon Diamond?

Light Balls are held by Pikachu, but you only have a 5% chance of actually finding a Pikachu holding one. You are doing it correctly, but you are having a case of bad luck.

The only time I've actually found a Light Ball on a Pikachu is in Pokemon Emerald in the Safari Zone. Perhaps if you have R\S\E, you could try finding a Pikachu holding the Light Ball in the Safari Zone and transfer it through the Pal Park. I don't think your chances are improved, however. Fights are just shorter in R\S\E.

This happened to me with trying to get a Lucky Egg from a Chansey - it took me well over 3 hours to finally find a Chansey holding a Lucky Egg. Just keep plugging away at it.

Also, CompoundEyes ability increases accuracy by 30%.

I got mine after catching ten Pikachus, and without using Thief only cuapturing each of them, it is supossed to appear on 1 of 20 Pikachus, so it is not that hard.
Put Pikachu as your first Pokemon and you are gonna get much more wild Pikachu encounters.
By the Way, Pikachu sucks, i have one with Light Ball, and Volt Tackle and believe me, is not worth it. Lux Ray owns it.

Yes your idea is good, pikachu has a 5% chance of holding a lightball and a 50% of holding a oran berry, all i did was catch 6 pikachu and i had 2 lightballs (lucky).

Well you need a pokemon with the ability Frisk (this ability can see pokemon held items), pokemon that hold this are Stantler, Shuppet and Banette. Compoundeyes has 2 abilitys to raise accuracy by 30% and out of battles encounter pokemon with items more often.

Hope this helped
Mip

Safari Zone.