It's kind of like the "Highlander;" there's only one. No need to bother worrying about 'best' or 'inexpensive.'
Alumnicon, by King. http://www.kinginnovation.com/category-pages/products/alumiconn.php
The purple wire nuts are not really suitable for long-term use. The Canadian products are not approved for use in the USA.
Keep in mind that many boxes -especially in trailers- do not have enough room in them for both the device and the connectors.
Also be warned that many aluminum wiring jobs use an undersized (#16) ground, one that is crimped where it enters the box. As a result, the ground will often break when you remove the device, and you have nothing left to connect a new pigtail to.