Aji is simply the Spanish word for pepper. The peppers in your Peruvian recipe are probably aji amarillo (capsicum baccatum). They have a fairly distinct flavor, not at all like jalapenos. If you can't find fresh, look for paste. It comes in a jar and should be available in most areas of the country.
"I’m going to break one of the rules of the trade here. I’m going to tell you some of the secrets of improvisation. Just remember — it’s always a good idea to follow the directions exactly the first time you try a recipe. But from then on, you’re on your own." - James Beard