There are surprisingly many books, posters, flashcards, lessons, research transcripts and recordings in Arapaho available online. They are mostly distributed free to encourage learning by approval of the Arapaho people, or they are held in academic archives where they remain free on principal.
Free PDFs of resources hosted by CSILW are here: http://www.colorado.edu/linguistics/csilw/alp/Additional%20Resources.html
If you want to purchase a dictionary, a college workbook or CDs of stories with written transcriptions and translations, sold as cheaply as possible (zero profit) by CSILW, email Cynthia Clark at firstname.lastname@example.org for a list of what’s available and prices.
Here’s a fantastic site with sound files for your mobile phone by the Cheyenne and Arapaho Language Department in Oklahoma. The sound files start at level 2 of 3, here: http://calanguage.com/arapaho-language/arapaho-level-2/
This site by Eugene Ridgebear Jr. has songs for adults and kids, animated stories and reading lessons: http://www.arapaholanguage.com/stories.htm
A whole ton of flashcards that will teach you to read, learn verbs, nouns and tons of other stuff, created by Arapaho teacher Mike Ridgebear at Wyoming Indian High School: http://www.studystack.com/users/ndncaveman
A completely different, older site full of Arapaho lessons and stories hosted by CSILW is here: http://www.colorado.edu/linguistics/csilw/arapahoproject/language/index.htm
Lots of links to learning materials, samples of language and Arapaho history are here: http://www.native-languages.org/arapaho.htm
An incredible archive of video, audio and printed documentation in old-school Arapaho has been collected by the University of London, and while you do have to create an account and log in, the resources are ample and of the highest quality: https://elar.soas.ac.uk/Search/Results?lookfor=Arapaho&type=AllFields&view=list&limit=10&sort=format_sort+desc%2Cclean_title+asc