How can I buy or download a horse cart

There isn't a a premium content horse cart.

If you saw something like that, it was most probably a cc deco item. 

Did you try googling Sims 3 horse cart ?

o ok I saw on the sims 3 site Exchange and then the category HORSES than animals that sometimes horses were exchanged with a cart behind it

That was probably just someone that replaced the standard pictures with one of their own and they used cc to achieve that.

are there any other possibilities to come to a horse cart horses?