on my 2000 honda civic DX my coter pin keeps breaking which causes my nut to loosen and come off and im tired of my A-arm dislodging what can i use that is strong enough to keep from breaking

 

This is a little difficult to believe.

If the tapered surfaces of the stud and the hole in the arm are in good condition and the nut is tightened properly, it's not going to come off-- with or without the pin in place. The pin is just an extra measure of safety.

 

Got to agree with the above^^^ also that has got to be the cheapest cotter pin I have ever heard of, 94

 

I buy cheap cotter pins, as long as the nut is tighten properly than it should not back out nor break the pin.

 

Sometimes I see people torque a ball joint nut to spec and then back it off to line the cotter pin up. If that is what you're doing, don't do that as it basically means resetting the nut to zero torque as soon as you back it off a bit.

You should always keep tightening if you need to rotate the castle nut to line up the cotter pin with a slot.

 

Post pics.

Is the car's front suspension modified or bone stock?

 

Thread locker on the castle nut.

Other possible explanation is that the ball joint is not properly seated or installed, or the incorrect ball joint is being used

 

As others have said i think there's something else going on here. Are you sure the ball joint threads aren't stripped causing the ball joint to slip out of the nut, shearing the cotter pin, and ultimately allowing the balljoint to come out of the spindle?