I need a stirrup for a spring rocking horse.

You would need them made from scratch

you need to be a little more specific in your question, actual strir-up ajustments or how to make sure they are the right lenght for your leg.

Traditional rocking horses are made from wood but nowadays you can buy them in colorful plastic and soft fluffy material, so it really depends what kind you like.
Go to a really big department store and check out their selection or you could just look online at Toys'rus to get some ideas.

Rocking horses have a bit of a reputation for being dangerous because of a few accidents involving the reins around the rocking horse. You do really have to be careful not to leave young children unattended.
And you can shorten the reins on the horse so that there's no danger of strangulation.
when kids go too fast on a rocking horse they can cause it to tip over but this is true of nearly all motion type toys.
i still think the rocking horse is a classic but you should be careful to supervise kids who are using it.

I'm from Brazil, so a have a toda's kids rocking horse end a need those four rubber bands. How can I buy these replacement rubbers?

Okay, we just replaced the bands and this is what we found. Work on them diagonally, i.e. replace right front then left back and so on. To remove the yellow caps you need to move the red plastic stirrup so you can uncover the screw. When you remove the screw it allows the yellow cap to come off. Do the same on the back bands by turning the red piece. Hope this helps. God Bless!