DSArms offers an AR15 semi-auto Ambi-Safety for $28 dollars; part number ZM41367AMBISA-A. Both parts of the Ambi safety appear to be cast. The groove allowing rotation is cut all the way around the DSA ambi safety, allowing the safety/selector to move into what would be a Burst/Full Auto position on a select fire rifle. However the rifle will not fire while the safety is placed on this position.
The DSA safety is one of the cheapest options for an ambidextrous safety for the AR15. The both sides of this safety lever are full size, so it may bump into your trigger finger when you flip the safety. This safety also sticks out a little more from the receiver then other safeties. This is most likely to ensure compatibility with the wide variety of aftermarket pistol grips available for the AR15. I really do not like how the safety will spin into the third position. If I push too hard or too far when trying to take the rifle off safe, I can push the safety past the fire point and any point past that the rifle will not fire.
I do not recommend the DSArms ambi-safety because it will rotate past fire. However if that is not an issue for you, the DSArms ambi-safety is one of the cheapest ambi-safeties around.