Aji amarillo (yellow) (capsicum baccatum) chiles are from South America, predominantly Peru. This beautiful yellow chile is quite hot with a slightly fruity flavor. Use the aji in salsa, ceviche, sauces or pickled.
In addition to whole chiles you can find aji amarillo paste as well as the chile in dried, ground form. These both make a good substitute if you can't find the whole dried chile.
Aji amarillo in the whole dried form is harder to find than many other types of chiles. You can find them (sometimes) online at Amazon.com. You may be able to find them in some small specialty stores that cater to the Latino / South American community.
The aji is rated at 30,000 - 50,000 units. View a list of Scoville and relative heat units for fresh and dried chiles.